Whats wrong with this guy?

okay, so... im a male teen, i go to high school and in cl, sometimes this dude comes up to me and acts all nice (fist-bump, handshake, some stupid comment), but then as he goes back to his real friends in the back of the clroom , he acts all thuggish and "cool". i dont usually reciprocate such social enthusiasm as he does, i seldom speak to him (im an introvert, you see...) can anyone tell me whats going on?