My guy friend goes out of his way to mention his girlfriend in every conversation we have. Why?

There's this guy I'm good friends with who is the same age as me (18), we've been friends for 4 1/2 years and we both leave in 5 1/2 weeks for the same college. We dated last summer for about three months starting the weekend of July 4th. After three months he just suddenly started ignoring me, he wouldn't text me, or answer the phone and when I would see him at church he would walk the other way, next thing I know he has a girlfriend at school who is two years younger and skinnier and prettier. I backed out gracefully without a word rather than ruin the little bit of dignity and pride I still had, but I still love him. About a month ago he finally stopped ignoring me and acting weird and started talking to me sometimes the way he used to do back when we were just friends. Within the last week and a half though he has been going out of his way to bring her into every conversation we have, Any time I'm around he says something about her, whether it really fits with the topic of conversation or not, it could be just because he hasn't seen her since school got out about a month ago, I don't know? Maybe he misses her? But at the same time lately he seems he has been going out of his way more and more to hang out with me, doing things like coming and finding me at church just to talk or to ask me a question that anyone could have answered, Coming and sitting by me when our families go out to dinner (which they do about once every two weeks or so) when used to he would go and find my brothers and hang out with them. Last week we went to summer camp his brother and sister and my two brothers and both of us and some other kids from our church group, and while their he would come and sit by me during softball games while other teams were playing or between his turns to bat, and he sat by me or as close as he could to me for classes a couple of times, One day (at camp) this like 14 or 15 year old kid started hitting on me and he went BALLISTIC! yelling at the kid to stop hitting on me I was too old for him and he couldn't have me anyway (scared the kid to death) Yet it seems the more he goes out of his way to spend time with me, the more he goes out of his way to talk about his girlfriend, What's up with that? I've asked people I know and their opinion is that with not being around his girlfriend he is feeling the attraction to me Again but that makes him feel guilty because he has a girlfriend and because of that he feels compelled to talk about her. I don't know, I just thought it might be good to get some totally unbiased opinions.