Your MIL is abusive. Not toxic, abusive. She gaslights to maintain control. She tries to alienate SS from his dad. She says very hurtful things.

How do you handle abusive people? You cut them off. You keep them away. SS has ZERO business being in the care of ANYONE who bad-mouths his father. So, your FH needs to cut this all out by cutting his family off.

They will come back and say that you're abusive and the reason why they can't see their grandson anymore, but the truth is that their ABUSIVE (I'm going to keep drilling that word in their) behavior has ruined their relationships with their son, grandson, and you.

And if your FH isn't willing to cut them off, then you need to reconsider whether this relationship is worth the abuse.

Your MIL says her grandson is her son?  WTF.  That is so not right.

How does he not know what his own CO says? Why is he relying on his mother to tell him what is in it?

Sounds like your SO is a bit of a Mama's Boy. Stop discussing it with her and move out on the appointed day. Then set very clear boundaries on her going forward. 

I wrote your FDH off when I got to the part where you said FDH and FSS did not move in with you because FMIL said the CO would not allow them to move out of the town that your FILs live in.

How much of an idiot, adult failure, failed man, failed father, and failed mate does this dipshit have to be to not know what his own Custody/Visitation/Support order for his kid says and has to have his mommy tell him what is in it?

What about this abject failure of an infantile adult semi-man is so appealing that you want to marry him?

Please, tell us.

 If you do not end this travesty of a relationship now, you are going to be at this Hags beck and call for the rest of her life and neither your FDH, nor his kid will ever be normal autonomously functioning adults even once this evil Hag of an FMIL kicks the bucket.  Guess who will have to change their failed man diapers and facilitate their juvenile crap then?

Two guesses and the first one does not count.

smh

Both you and your bf have made a lot of mistakes, and some of them are going to continue to cause problems for years to come.

You both were wrong to move the relationship along so quickly. Your bf should be very cautious about who he allows in his son's life, and you should be very leery of dating men with kids. The general belief around this site is that you should date for apx two years before considering joining households. This gives both parties enough time to get through the "honeymoon period" of best behavior, really get to know the other person, vet the overall dynamic thoroughly and critically, and allows the kid(s) time to adjust. 

We've had other posts from women who became involved with single dads who abdicated their parenting responsibilities to their parents, and it seldom ends well. I find it troubling that you say your bf moved in with his parents in order to help them financially. Why couldnt he help them while maintaining his own home? And why has he allowed his mother to overstep and essentially become his son's pseudo mom? The answer probably involves being a bit of a mama's boy, taking the path of least resistance, and doing what serves him best. Now, thanks to the lack of appropriate boundaries his son is damaged. Then he met you, and quickly moved you into his parent's house. We all know there can be only one queen in a household, so his mother probably assumed you'd adapt to HER arrangements.

This all is DYSFUNCTIONAL and TOXIC, and you will likely be blamed for upsetting their sweet yet screwy arrangement. If it weren't for YOU, none of this would have happened, and they would all still be happy ... Yes, you will be the scapegoat, and odds are your bf won't be able to adequately defend you. In times of weakness, he'll resent you, and his son will resent and see you as the cause of all the upheaval and change, too. Worst of all, you've put yourself at the mercy of this woman by quitting your job. That's a hard lesson you need to learn ASAP - no matter how into a guy you may be, NEVER give up your career or independence.

If you TRULY want to set your relationship up for success, it's going to take some hard work, sacrifice, and backpedaling. You need to reclaim your independence by regaining traction in your old career and moving away from the dysfunction - ALONE. This is so you can see how committed your bf actually is to cutting those apron strings and being a functional single parent. HE needs to move out, make a stable home for him and his son, and wean his son away from toxic granny. ON HIS OWN. He needs to build a healthy foundation BEFORE adding a partner to the mix, and that includes creating healthy boundaries for his mom so she can enjoy being a grandma instead of a surrogate mom. All of that is HIS responsibility, and if he can't/won't do that, he doesn't deserve to have you.

I've said it once I said it a million times the in-laws are by far the worst part of step life. 

Your overbearing mild is not going to go anywhere she is always going to put herself between you your SO and SS. The fact that she calls her grandson her son just shows you how overprotective and possessive she is of him. so you must ask yourself what is she protecting him from? It's clear that she's protecting her grandson from her own son and from you. She is going to be very territorial about your presence in his life no matter what you do. 

The word "Mama's boy" came to my mind before I even finished reading your post. Do you want to feel like you're competing with your FMIL? Or feel as though your FMIL is your future husband's "other wife" ? Also, if your FMIL is already talking down to you now while you're engaged, more than likely it will only get worse once you get married especially considering how controlling, manipulative and territorial she's already showing herself to be.