Ginny, you are so good to deal with this the way you do!
Should you push him to go to the wedding? I think you're doing the right thing in convincing him to go. But ultimately, it's his decision. I guess he needs to talk about it with his son, and say how disappointed he is. And try to understand his motivations. Is it really to not make his mom uncomfortable? Did she even ask for this? Or is it because he has problems with the fact that his parents are divorced?
In any case, if it's at the mom's request, if your SO doesn't go and his son ends up being disappointed, it's very possible she will accuse YOU of ruining her son's wedding.
Your SO just won't win anything by not going, in my opinion.