RUN as fast as you can away from that situation. He has told you that it will never change, that mom will always come first (at least he was honest). Do you really want to live the rest of your life as second fiddle and furthermore, be put down by his family with no support from him? I sure wouldn't.
I know you are upset right now and you have a right to be - but what you need most is support from him and what you are getting is nothing, actually worse than nothing - you are being upset by him.
In the long run you will be much better off without him - even though this is a horrible time for this to happen..........
Good luck sweetie - you'll find Mr. Right (and he ain't it) - give yourself time!