Loyalty binds. I take everything my SD says about BM with a grain of salt. In general I believe what she says is the truth but she tends to embellish things to gain approval from DH. I also would bet she is talking about us to BM behind our backs. 
 

IMO you dodged a bullet by not having him live with you full time. Most (not all) SM's end up regretting it. In fact I actively encourage a relationship between my SD and BM for this very reason.

Love BM 

The bond these parents have with these kids is enmeshed and twisted. They will never care for me because I am not their mom, no matter what a piece of trash BM actually is. Ss now 17 did this at the beginning of covid and bm wouldn't respond to take ss back until we slapped her with court papers. Ss then 14 went back to BM and left us high and dry. I was glad he wasn't living with us but what a friggen hassle it is when there's drama in their house... we feel it. There's no control here except where BM is concerned. Ss17 ousted dh out of bed to say bm needed a loan for her dog vet bill and dh paid up! I suspect things like that are done because she's just showing me she's still around and can get what she wants. But in know they talk badly about us to bm but at the end of the day... its not in our control. As long as there is a room here for them, they will be welcomed. Bm kicked them out in November because she needed a break, didn't respond or take them back for over a week. Nah thanks. 

A very real thing. Similar to Stockholm syndrome. I've experienced this with my own bio children. My ex is an abusive narcissist. Until people seek either professional help or do some serious work on themselves they will always be affected. The amount of bs my kids put up with because of their "love" For their dad blows me away. They see him and KNOW what he is and yet there is this need to seek his approval. It's sad and heartbreaking to see. 

The dynamics in play in blended situations are real and definitely can not be changed easily. My DHs perception of the situation is polarized from my own. It was doomed for failure. I can't change his way of seeing things, and he certainly won't change mine. Dh also shares this trauma bond with his children due to the way he leaned on them through the divorce before I came along. The divided loyalties alone are extremely hurtful and I always feel like an outsider. But this is common for the role of sm but I certainly can not consciously overcome my hurt and dh seemingly can not control his anger towards me in his defensive response towards his children.  It's all kinds of toxic and bm is wretched... it is more stress than a first family for sure. I can understand the almost 70 percent fail rate for second marriages. Sadness is all that's left.

SS hated his mom when she left BF/SS for a married man. Gave up custody.  Came back after man went back to his wife and requested primary back and back CS (and got it!) after I was in the picture and poisoned SS against me.  He refused to have anything to do with me and I haven't seen him in years.  Told dad that he considers Mom's house as his residence.  That mom is a better parent then him. 

He now has recently been complaining to BF that he is a "negative person" because "I was raised by a negative mother."

Your SS probably truly believed and felt all of the bad things he said about BM at the time he said them. He was not lying - it was how he felt at the time. The pull of the biological Mom is a strong one. Once he was around her, all he could feel was the love. It is very common for abused kids to defend their abusive parents. Read around this site and you will see this dynamic repeated on a regular basis.

Your SS had no grand plan here - he is operating on a purely emotional level. It would be helpful for him to attend therapy on his own, without either parent present.

No matter how much toxic stench passes back and forth through that cord, it binds.

Sadly, it binds even when it should not.

meh

IMHO, 15 is too late to invest in a change of custody.  Once BM kicked him out, it likely would have been batter to just file for a CS ammendment with the new situation rather than try to go for full meal deal custody. Of course, hindsight is always 2020. Your DH had no way of knowing his son would get all tearfully mommy focused in the first minutes of a therapy session even after she kicked him out and he was ready to tell her to F-off and die.

Your SS is telling you what he thinks you want to hear, but kids always love their parents, no matter how horrible they are

What was your DH's response/reaction to his son when he shows up after trashing his mom's house?  

Did he punish his son for his behavior?  Did he agree with his son's reaction.. and agreed his Ex was treating his son badly?  Was he happy to have his son show preference for him after years of appearing to prefer his mom?  Did either of you state any opinions or say anything.. even just "agreeing" that could be seen as negative of the EX?

Because I actually am not surprised at all that the kid would be happy to push off responsibility for the estrangement onto someone other than himself.  He doesn't sound like the most mentally mature person.. still a kid.. and now that he is no longer mad at his mom... he doesn't want her to have any reason to be mad at him for being disloyal.  In reality.. the outburst of anger and being mad.. were not lies.. he was mad.. and likely hurt by the situation.. and angry at her.. but he can also be still her son.. and still love her.. and be hurt that she didn't try to fix things sooner.... but once she did.. the tears and the emotions.. he is just a kid who wants his mommy right? 

But.. bottom line.. it's not right for either parent to try to "win".. to sway their child's feelings about the other parent.  The kid has a right to have a relationship with both parents.. without worrying that the other parent is undermining.  

I don't think that BM's demands need to be agreed to blank check style.. BUT... I also think that your DH and his EX have a kid that could use some therapy.. support.. and possibly.. yes.. some family counseling.. though.. I'm not saying it has to be joint with your DH and BM.. they can have separate sessions of course.

50/50 sounds reasonable.. both parents should have the opportunity to have time.. and as long as that is logisticaly possible.. it's a good balance.  

And.. it would not be a terrible idea for both parents to be setting limits on his gaming/computer use.. it seems that has gotten to be a serious problem for this kid.. and if they can't agree together on those limits.. your DH should at least enforce some in his home.. and perhaps access to his gaming as a condition of meeting obligations like school work and therapy is not a bad idea at all.

I'm sorry  you had to go through all that.  I agree with other posters, the BM is always loved and preferred, no matter how rotten she is.  I hope he moves back in with his mother; he may turn on you both and if he's under your roof, that'll be a nightmare.

Take care of yourself, we're here for you!