Why do guys say rude things, then they're all "JK!!"?

Ahh! I'm 13 and this one guy and I went to the movies, and he put his arm around me, etc. He always texts/calls, he's pretty nice. But he always says REALLY mean things, then he is like "JK!!!" but it still hurts my feelings, he said he would stop but he isn't. Like he will say "You've gained a few pounds.." and i'm like "uhhh?" and hee's all "JK!" and it's annoying me.? I'm moving, so i guess it doesn't matter, but are most guys my age like this???