Two views from our house!
My feeling is that you accept not going to the wedding and plan a lovely day with friends. If you go you may make things worse, be treading on eggshells, etc. I think your husband can go in his own right. You can have a diplomatic illness, or preorganised family event.
My husband, a widower with three children when we met, thought that no one should be telling anyone what to do, but he thought that your husband might have a talk with his son, on his own, to try to find out what the real problem is, and find out how it could be resolved.
Hope the day goes well, no matter what happens ?