Why is my best guy friend acting like a d1ck?

so there's this guy who used to have a crush on me. nothing really ever happened between us (i didn't/don't like him back), but we became best friends last year, all the way through this year. we're both pretty nerdy, but we used to not care. i still don't care, but lately he's been following the "popular crowd" a lot. like there are a group of guys in my math cl who are the "y jocks" and he's in athletics too, and he'll try to hang out with him, and when i'm only with him, and they're gone, he acts like nothing happened, like he didn't just ditch me to try to talk to them, and will tease me again! It's so weird, and i don't act clingy or anything, i have my own friends, but what's going through his head, and what am i supposed to do?!