TLDR: He gets the side eye from me too.
I think it could go two ways.
1. He probably feels really bad about going with the flow as a kid. And now he actually has a backbone and stands up for what he believes. And he truly, genuinely wanted to be friends, so this really is a nice little reunion.
--OR--
2. Something bad/tragic happened that made him change so drastically like that. He developed something like a longing to stand out because he used to be so passive. And now in order to rid himself of his "pitiful" past, he forces a faux friendship just to prove that he's not the same person that he used to be. // I'm suspicious about how angry he got at the older classmate. Maybe it was righteous anger, or maybe he was projecting his past emotions/situations onto others.
I don't know how to feel about read head guy like he seems cool, but I am also very suspicious of him mainly because most of the times( almost always) characters like that turn out to be just disgusting lil' pieces of !SHIT!...BUT...maybe he'll be a nice one...I don't know.... WE'LL SEE...