Try to get to know him on a friendship level before approaching him with romantic intentions. get to know him outside of his appearance. know what he's like on the inside. start out with small talk every so often, find out his interests to see if you have one in common, if so, ask him about it.
Try your best to stay calm when around him, he's a normal person just like everyone else.
If he has a girlfriend, just stay his friend for the time being. you want him to be happy correct? then be patient, and I can't stress this enough when I say be his friend first.
Don't forget, you're still young, you have the rest of your life to spend with your one true love, how bad is it to wait a few weeks or years for him to love you back? when you have all of eternity.
Guys can't read clues, so after you become friends with him, get to know him really. then you need to just tell him.
Remember, this takes time, just be patient and you're probably a great person, so, he'll love you back once he gets to know you.
Good Luck!