I've not been in your particular situation, but have known people who have had this happen to them.
Some guys are afraid or don't know how to deal with a relationship once it deepens. Instead of things flowing naturally from one stage to the next and him going with the flow, he is probably afraid that you will wan't to push for commitment in the form of a future together (eg: engagement, wedding, babies). This scars the hell out of some guys and they don't have a clue how to deal with it.
I don't know what your ages are, but maybe there are still things he'd like to do with life prior to settling down. Often guys who don't have sisers or can't express things to their mothers will learn to bottle things up inside b/c they don't know how to communicate with their g/f on this particular matter. A lot of guys will ask advice from sisters and often mothers regarding relationships and g/f's.
If he wants to come back, tell him the two of you need to have an honest chat. Let him know exactly how you feel. If he becomes a yo-yo and breaks things off then comes back constantly, forget him. That's not healthy at all and you just end up on an emotional roller coaster.
If he doesn't know what he wants, then he needs to leave things alone until he possibly matures a little more, or knows where he stands in life and what he wants out of life. All the best