Lulu, I'm in the same situation that you, except it's the most classic one in my case and DS14 who suddenly doesn't want to go and visit their dad.
Like your SS's mum, they've never build a strong relationship. His dad is very self-centered and has never really shown much interest in him as a person, and has never asked him anything about his life outside of his own world. He is very family oriented, which is great, but he considers that he (and his sister) should be as happy to come and see his partner and her children as him.
I did see it coming and frankly, have been surprised that DS has continued to go regularly until recently. They never ever do anything outside of home and chores, and children are left to entertain themselves completely, which wouldn't be so bad except that there isn't room to do so and the kids are different age/gender, which means that even though they get along well, they don't have a lot in common.
DS never gets to do things with his dad alone, and even when they do spent a bit of time together, they never have meaningful discussions. To add to this, DS, although sociable, is the type of kid who has always needed his own space to retreat. As a teenager, this has become even more important to him and he can only cope so long with group socialising. Unfortunately, not only he doesn't get that space there, but he is critisized for it. I think they are all the opposite, love being with others constantly and hate being alone, so they struggle to understand why he feels the way he does and they take his wishes to retreat as a rejection. This has lead to DS gradually feeling guilty yet their attitude also makes him feel rejected.
I have told him to speak to his dad about it, but he says they don't have that sort of relationship where they can talk about things, and I do understand because indeed, that was a problem with my relationship with him as he never wanted to 'talk' about things, always avoiding the subject.
The thing is, I am genuinely devastated for my son that he feels the way he does and would truly love for him to have a good relationship with his dad and his family. I would much much prefer that he wants to go there, but I do understand how he feels and as such, wouldn't want to force him. I've spoken to him about things could be made better, but I don't want to preach and I know that there isn't much he can do to change the situation.
I could get involve, try to speak with my ex (who I haven't spoken to for many years) or his partner (who I've never spoken to), but I know it will be futile because I'll face the same response than my DS is getting, ie. nothing is my ex's fault, it's all my DS, only he should be making changes in his attitude, and somehow, the conversation will end up with it being all my fault too!
The situation was predictable and I really do believe that the only resolution would be for my ex to actually question why our son is refusing to go and self-reflect on what he could do make things before, but that would mean him taking responsibility for the situation and however much I'd hope he did so, I very much doubt it will happen.
So I won't force my DS to go, nor will I get involved. I will continue to encourage him to try to speak to his dad, and if my ex were to contact me to ask me my advice, I would genuinely try to help, but otherwise, all I'll do is whatever I believe is best for my son, and that's to make sure that he feels no pressure to go just to appease his dad and the guilt he puts upon him.