Need some advice or opinions on problem with my friend, please help!?

Been best friends with this guy since 6th grade, so about 13 years. About six months ago he started acting different. Not quite sure how to describe it, he just didn't act the same, wasn't into the same stuff, wasn't into the stuff we used to do, didn't behave like he used to around me, etc. He started becoming more distant and distant and eventually I didn't really hear from him anymore. I also noticed he had some new friends. I called him up and just said hey, sorry if I did anything to offend you or whatever...he played it off as no no, i've just been busy with work, but it was total BS. He will on occasion still go out to the bar or whatever with me and our other good friend (and it seemed like he had fun) as recent as a 2 months ago, but now won't answer IMs and says he can't go out when we text him. I'm just curious what happened, if we grew apart or why he would just change out of the blue? Any ideas? How long do I try to make amends and see him before I just give up?