To me, this just sounds like a phase.
In your teenage years, your hormones are out of control. Its a fact. So, maybe, he just thinks hes gay. Plenty of guys do that, sometimes when they have trouble finding an attractive women who finds them equally attractive, others when they are confused about sixuality and who to be attracted too.
I would wait it out. Chances are; He probably isn't gay. Its just his hormones are out of control, which is perfectly normal. If he is gay, however, and after a couple weeks he is still chasing you, you'll need to lay down the law. You can't just let him continue with what hes doing if you don't feel the same way.
So go with his old andy's advice; Punch. Harder.