He Wants To Postponed The Wedding

My fiancé wants to postpone our wedding because his son was sent to a mental hospital. His son is kinda a troubled teenager. He doesn't do anything bad exactly, he's just kinda rude and acts like he hates everyone including his sisters. I ended up walking in on him in the bathroom burning his thigh with a cigarette. He has burns all over his thigh. I told his father and his father asked him what was wrong and he starts yelling and he cusses me me out for telling on him. It was just a lot. He started breaking stuff in our house and it was a nightmare. My fiancé didn't know how to help him. His friend recommended this place thats like a mental hospital but isn't exactly one for people under the age of 18, so he sent him there. My fiancé talks to him on the phonw every night and he is supposed to go to family therapy there with his ex every Wednesday. So my fiancé is now saying he wants to postpone the wedding until his son gets out. If he does well he will be out in a month or so. He said it won't be the same if all the kids aren't there. Tbh I don't even think his son likes me so... Anyway I've tried talking to him about it on several occasions but he is still saying he wants his son at the wedding and he doesn't want to get married without him being there. I dont want to push the wedding back two more weeks, but he won't budge at all.