Well it's all his choice, but if she's an abuser then it could be that she's thoroughly cleverly manipulative. This happens, as we all know - usually a male abuser and female victim, but it happens this way round too.
Reasons for not returning 'home' to see mum/child, for letting down friends, for reneging on promises, these things are very normal behaviour from an abuser's victim. If you question them too hard, push them too far, they will stop seeing you as it's easier day to day than trying to explain. Most things the abuser does are so small, apparently insignificant that tryong to get anyone to see it is virtually impossible -or you think it is. They also tend to undermine you, tell you that your friends all think you're mad or bad, they make a fool out of you if you try to involve anyone else and generally make your life a misery, especially if you're also financially dependent on them; that's something they will try to bring out fairly early on. Isolation and financial dependence are basic tools of the abuser and once that's happened, well the victim has little chance.
I hope it's not abuse, but it wouldn't surprise me at all if it were.
He needs to spend some time in a normal home without her. Can you feign serious illness and desperately want to see him? Well, no, probably a bad idea, but try to think of some way to get him staying with you or your mum for a week or two and see how he goes.
AIBU is not the Board to post on for this. Try Relationships, and see if anyone there can help you with ideas on how to find out what's really going on, if he is a victim of abuse.