however, it seems like he's decided that he can only be close to one person, his fiancee

He's young, he's in love (I hope he is, anyway) - that's the way it is for now. Plenty of time in the future for family relationships to settle down on a more even keel. Just give it that time. Being a single Mum will make it harder for you but, if he's been a good son to you up until now, there's no reason why he shouldn't be one in the future.

Wait until the grandchildren arrive - son and DIL will both want you then! :-)