Why is he doing this? Please read the story!?

I am a guy myself, and I wouldn't do that myself but, it sounds that as his girlfriend was away, he was probably lonely, and having you around probably took away some of that loneliness, and he most likely developed some feelings for you, and lost a little interest in his girlfriend; but now that his girlfriend has returned and he has seen her again, he probably regained his interest in her again and probably feels guilty about what he did behind her back, so he may be ignoring you for the time being to forget his feelings for you so that it does not ruin his relationship with his girlfriend. It isn't right or nice what he did, but it sounds like that to me.