We both DO NOT want him to move in with us. He fights constantly with his mother-in-law. She is 82 and has dementia so that doesn't help the situation as well. We both don't know what to say to him because if we flat out say no he will start world war 3. Any advice on how to say no would be awesome.
It was kind of a stupid slogan regarding drugs, but it fits this situation: "Just say No."
Don't set a precedent this early that will haunt you for the rest of your relationship. It's good that you're both in agreement about it.
Iam801, stick to your guns. He has no right to expect to move in with his son because he has a caregiving situation with his MIL that is difficult. If he wants to move, that is his perrogative, but also his respoonsibility. If MIL is in her 80s my guess is he is in his 50s. Difficulties with his MIL are HIS marital issue, not your problem.
Leave it at that. If he keeps wanting an explanation, you could say you are sorry, but it just isn't possible.
I wouldn't care, if he does get mad. He is giving you an unreasonable request.