Either he was too embarrassed of his family to let you meet them, or he was too embarrassed of you to introduce you to them. If his mum likes a drink or 30, most likely the first. After 20 months together though, did you think it through and especially, talk to him about this? If he knew how much it was affecting you, he may have let you meet them regardless. They could be THAT BAD that he was simply afraid it would change how you felt about him.
Unfortunately there is also another option here. You don't say how old you both are, but is there the possibility that he has a family - wife, kids - uptown and you on the side? Would his job be the sort that he could easily explain being away from the family for several nights each week? Was he always available for holidays etc. like Valentines, Thanksgiving, Christmas, Easter etc. etc. If not, here is your answerand you have done the right thing. If he was there to spend these with you, maybe you would like to reconsider dumping him, because that would suggest he also has your best interests at heart. Or maybe his family is obscenely rich and he wanted to be sure you loved him for himself, not his wallet!