Apologizing in advance for the length...but this isn't even half of what I've been dealing with!

A little back story to set the scene. DH and I have been together just over 10 years. Four children (16,16,16, & 15) between us, none together, that have lived with us FT for 9 & 1/2 years. The first two years were exactly what I expected, skids & bios fought, DH & I argued. Then around the three year mark, things started to fall in place for everyone, except SS. At first the issues were him lying to get the other kids in trouble or himself out of trouble. Then the issues spilled over into school. *Sidenote #1 SS was diagnosed ADHD and medicated. However, even though it helped with him retaining lessons at school, it did nothing to help with his lack of motivation or his sneaky personality (throwing away homework before getting home with it). I thought I was doing the right thing, DH worked out of town Monday-Friday, so I took on the responsibility of handling school issues with SS. *Sidenote #2 BM is an addict and is not consistent in visitation, and certainly not capable of caring for SS for any length of time. However, once DH started being at home in the evenings & SS was getting older and into more trouble at school, I completely disengaged from any & all things school related. From that time to now, SS has had multiple school write-ups, out of school suspensions & in school suspension. For every referral/write-up SS came home with a far fetched scenario or excuse or out right lie about how he wasn't at fault. DH did try punishing SS in the beginning, but after a while and SS continuing to get into trouble he gave up. There was/is an issue that DH can't seem to get passed. During the fifth grade, the school did a week long anti-bullying thing. From that point on, any time there is trouble (and it's only mentioned when he is in trouble) with SS, that is an excuse SS & even DH uses to excuse his behavior. "No one likes me, everyone picks on me". He is manipulating DH. DH still believes SS when he uses this excuse, even though multiple teachers over the years have told him that SS is a leader and the other students follow his behavior, he has many friends at school, and if he would stay out of trouble and not disrupt the instruction time the teachers would love to have him all day.
However, in the past year SS has found another form of manipulation. Claiming he is going to kill himself every time he gets into trouble. Sidenote #3 He has been evaluated by a psychiatrist & a psychologist both who have told us this is a form of manipulation and a way to avert the attention from what SS did wrong and any punishment he may have coming to cause DH to fear SS reaction. Basically it's SS way of assuring DH doesn't punish him. Recently he was suspended from school for an act of vandalism. DH's "punishment" for SS was he isn't allowed to go anywhere. SS was allowed to keep his cell phone, television privileges, Xbox, free/play time outside in our yard. A day or two ago, DH and I were texting about SS and somethings he had done at home that I felt DH needed to address. Somehow someway DH managed to make the following excuse for SS "He really doesn't have any friends or anyone to do anything with, and he just can't get passed it." My first thought to that text was - "Not true, he talks about these 5-6 friends all the time, DH just doesn't want to have that excuse for SS behavior taken away" my second thought was "If you really believe that, then how was his "punishment" of not being able to go anywhere a punishment at all???" The older children in the home either drive or have friends that drive. SS is nearly 2 school years behind than the others, so he nor his friends drive...maybe that's why DH feels like SS doesn't have friends, IDK? DH also texted that everyone in our home gives SS a hard time. My response to that was that the other kids are angry,they may take it out on SS because they know better than to be disrespectful to you, because of the differential treatment you show SS vs them (them being my 2 bios and my SD). At any given time, DH will say something to one of the three older children for something they have done wrong but SS can do the exact same thing and DH doesn't say anything to him. We have a situation that came up this morning. DH came to me and told me I needed to do something with BS for what he did. LET ME SAY, I AGREE WITH DH. I have already addressed it with BS and he knows there is a punishment coming this afternoon. My issue is that exactly a month ago from tomorrow, SS did the exact same thing that BS did last night, and DH was told SS did it...but SS was not punished, not even a word has been spoken about SS doing this thing that DH wanted me to punish BS for. DH & I have had this conversation before...the difference he shows SS vs the others. The other three children, SD his own daughter included, complain to me about it all the time, but are not willing to say anything to DH anymore because he says he doesn't show a difference. Says that he is constantly getting onto SS. That's not true, but if it were true it would be because of SS behavior and no one else's fault. Also, even though he does "get onto" SS sometimes, it simply is not enough. This is a kid that has been in so much trouble over the years, a criminal act of violence last year & an act of vandalism this year - that's just the two out of school suspensions. Not to mention all the things he's done to get in school suspensions, problems in our home, problems with his stepdad when he did go visit his BM, I could go on and on and on. SS is literally the only source of tension between everyone in our home. Otherwise, DH & I are great. The other three kids are great at school and home, and when they screw up (as expected all teenagers will) they accept the consequences and move on. Part of me says 15 years old is old enough to know exactly what he's doing and if he doesn't want to change, he's just one of those kids that's going to end up in jail/prison. The other part of me says 15 years old is still young enough to get through to him, but how - especially if DH doesn't think he'll ever change??? I know I am just SM, but this is my DH's kid, it's going to have an effect on our lives forever. But when do I know there is no hope left for turning him around & it's a lost cause?