If his mother signed off her rights how is it she still have visitations? No court would give visitation to a parent whose parental rights were terminated (voluntarily or mandatory). 

I don't understand how Chris avoided jail - child abuse is child abuse, no matter who commits it.

Is your SS in therapy or has he been? Can you talk to a therapist about how to handle these situations. Abused or not, life will still have to happen and expectations aren't a bad thing. I understand not wanting to trigger him or make him feel unsafe, but grounding him because he didn't clean his room isn't abuse. It isn't too harsh or cruel. It is an expectation that if not handled has a consequence. 

I think a therapist would be able to help you navigate appropriate and natural consequences for him that will still make him feel safe and loved but accountable. We all want our kids to successfully launch into adult hood but the current track he is on will create an emotionally stunted adult with no ability to self correct. 

Being undermined as a step parent sucks. Your taking on responsibility for another child, doing all the things I assume (laundry, meals, transportation, homeowrk, talking about things that may bother him, gifts, special treats...etc) but you get none of the say with that child. Sometimes for me it almost feels like Im expected to do EVERYTHING for my SD but am constantly reminded that Shes not mine and that dad has the final say on everything, even though Im the parent that spends the most time caring for her. Its not fair. 

I dont have much advice cuz DH and I still struggle with this sometimes. Just keep trying to talk to DH about it, and remind him that you do the things for SS that you do, to help make your DHs life easier and instill that yall are a family. Maybe if you dont get a say, dont keep doing some of the things to help DH out. make him carry all the responsibility for a while. Its not right for everyone, and I cant personally bring myself to do it (Im a control freak) but maybe you can? Good luck Mama and remember your doing better than you think you are. 

Specifically with chores and school. I don't have a FIL living with us, but I am often made out to be the villain if I ever get upset that skids aren't doing the chores they are supposed to be doing. I too had a SM who was absolutely awful to me, and I had WAY more chores than these kids do, but I don't hate or resent my SM for making me do them, I actually think it was a positive thing. I also cooked a lot for myself, and I was proud of what I could do as a kid. Skids here have one measly 5 min chore Mon-Wed, clean their rooms MAYBE once every 2 months, and do their laundry (1 load a month). Honestly, skids are fine with the chores since they have been in place for 7yrs now and they are used to them. SD13 does them automatically without complaint. SS16 sometimes does them without a reminder, sometimes tries to act like he forgot in an effort to get out of them. It's when we added in an additional wknd chore of unloading/loading the dishwasher that has caused WWII in our house. A few suggestions that helped this become more of a set rule in our house that might work for you-

A quick Google of "are chores good for kids" will bring up article after article of why YES, chores are good for kids. I would print one and have your DH and FIL read it.  

Next, Make a chore chart/house rules list that is visible to all in the home. Cleaning his room, and several other chores can be added. When I did this, I engaged DH and had him pick the chores himself. I took a recent course in a SM seminar, and it suggested sitting down with DH and SS (and maybe FIL) and going over these things to ensure everyone is on the same page. It could be reiterated that these are SS's chores, to be done alone. Include "rinse your dishes", be quiet when babies are sleeping, etc. 

Your DH is not doing SS any favors by treating him differently due to past abuse, but this is a harder one to get through to them. My DH also expects skids to do well in school, but when they don't, he will not enforce the punishments he has threatened. They are just setting skids up to be terrors as they get older, and I can see SS16 thinking he doesn't have to listen to DH now. I wonder why- nothing is ever enforced, he doesn't take DH seriously half the time. There are also plenty of articles on that online also..."guilty parenting"... 

Correction without connection breeds resentment.  Your husband needs to be the one setting standards with his son.. and setting consequences.  Sure.. he can ask your opinion.. but a list in your writing on the wall? nope.. recipe for resentment and disaster.

And.. you have a DH problem... he abdicated his parental responsibilities to someone else.. his father.. who now feels he has a right to weigh in on how the child is raised/treated.  

Your husband needs to want the changes you want.  It sounds like he doesn't care as much as you do. is he also laid back on parenting your other joint children?

So.... you need to figure out what your boundaries are.. filth.. ?  clutter?   messy room? Pick the stuff that impacts you..  So.. maybe you don't worry whether the kid makes his bed every day.. or isn't great about putting away clothes or whatever.. but he cannot grow science experiments in his room.

Make those hills to die on your husband's problem.. if the boy doesn't do it.. your husband does it.  not you.. him.

Focus on the facts, reality, and behaviors.  If SS makes poor behavioral choices, address them in real time with effective consequences.  His experience with his Bio Mom and her clan are irrelevant to that.  Set the standards of behavior and standards of performance and hold him accountable just as you would you own children. Insist tthat your DH enforces the same.  If he is your true partner and worthy to be, he will step up.

IMHO your DH and his dad were dipshits in giving a young child the choice to see anyone toxic and who served him up as a victim to a child abuser/wife beater.  Even it that POS is his mother.  What viable adult/quality parent abdicates their adult position to a toddler to make their own decisions?

Though nowhere near as severe as your blended family situation and the high risk situation your SS is exposed to on the shallow and polluted maternal side of his gene pool, we did have to battle a toxic and manipulative SpermClan for 16+ years under a CO raising my SS-30.  My DW had full physical and legal custody of SS from birth.  The SpermClan had 7wks of visitation per year. (5Wks summer, 1Wk winter, 1Wk spring)  Even with that limited long distance visitation, they tried to manipulative, and PAS SS with lies, etc....  So, we introduced him to the facts in an age-appropriate manner.  As he got older and would come home from SpermLand visitation upset that his "mother was making his 3 younger (spermidiot spawned) half sibs by making the SpermClan pay CS" etc... we provided SS with clarity with the facts. Ultimately we reviewed the court rulings, Spermidiot's arrest record, etc, etc, etc...... By the time SS was in his tweens and through his teens he was doing his own research in our custody/visitation/support files when they would spout shit that did not pass the smell test. The last 3-4 years under the CO he was calling their crap in real time when th would lie, manipulative, etc....   They did not like that at all. SpermGrandHag lost her proverbial shit over SS knowing the facts which gutted their ability to manipulate SS and lie to him. For the most part.

Kids need the facts. The need them to protect them selves from the toxic half of their family, if so cursed, and they need them as adults. These toxic people never stop. They are insidious.  If the kids do not have the facts and have the support of the qualith side of their blended family they can overcome even the toxic.  

My SS is now 30, has put his SpermClan far behind him for the most part, and is thriving as an adult, a professional, and a man.  His 3 younger half sibs are sadly failing in adult life.  Spermidiot spawn #2 is on the dole, #3 is in prison, #4 is not too far behind #3.  It is heartbreaking for our son and for his mom and I.  Though he understands he is far better off with limited interface with any of them.  My SS and his sister detest their Spermidiot. #3 and #4 worship their father. Which is why they are either convicts or prison bound depending on which one is in question.

Protect your family. Including SS.

Good luck.

Focus on the facts, reality, and behaviors.  If SS makes poor behavioral choices, address them in real time with effective consequences.  His experience with his Bio Mom and her clan are irrelevant to that.  Set the standards of behavior and standards of performance and hold him accountable just as you would you own children.

IMHO your DH and his dad were dipshits in giving a young child the choice to see anyone toxic and who served him up as a victim to a child abuser/wife beater.  Even it that POS is his mother.  What viable adult/quality parent abdicates their adult position to a toddler to make their own decisions?

Though nowhere near as severe as your blended family situation and the high risk situation your SS is exposed to on the shallow and polluted maternal side of his gene pool, we did have to battle a toxic and manipulative SpermClan for 16+ years under a CO raising my SS-30.  My DW had full physical and legal custody of SS from birth.  The SpermClan had 7wks of visitation per year. (5Wks summer, 1Wk winter, 1Wk spring)  Even with that limited long distance visitation, they tried to manipulative, and PAS SS with lies, etc....  So, we introduced him to the facts in an age-appropriate manner.  As he got older and would come home from SpermLand visitation upset that his "mother was making his 3 younger (spermidiot spawned) half sibs by making the SpermClan pay CS" etc... we provided SS with clarity with the facts. Ultimately we reviewed the court rulings, Spermidiot's arrest record, etc, etc, etc...... By the time SS was in his tweens and through his teens he was doing his own research in our custody/visitation/support files when they would spout shit that did not pass the smell test. The last 3-4 years under the CO he was calling their crap in real time when th would lie, manipulative, etc....   They did not like that at all. SpermGrandHag lost her proverbial shit over SS knowing the facts which gutted their ability to manipulate SS and lie to him. For the most part.

Kids need the facts. The need them to protect them selves from the toxic half of their family, if so cursed, and they need them as adults. These toxic people never stop. They are insidious.  If the kids do not have the facts and have the support of the qualith side of their blended family they can overcome even the toxic.  

My SS is now 30, has put his SpermClan far behind him for the most part, and is thriving as an adult, a professional, and a man.  His 3 younger half sibs are sadly failing in adult life.  Spermidiot spawn #2 is on the dole, #3 is in prison, #4 is not too far behind #3.  It is heartbreaking for our son and for his mom and I.  Though he understands he is far better off with limited interface with any of them.  My SS and his sister detest their Spermidiot. #3 and #4 worship their father. Which is why they are either convicts or prison bound depending on which one is in question.

Protect your family. Including SS.

Good luck.