What's goin on with my best friend?

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 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 would on occasion still go out to applebees with me and our other good friend, as recent as a month 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?