just tell him (boys like it when girls tell them they like him)seriously
If he already has a girlfriend, likes this girl a lot, and they are comfortable together, then you should not be trying to get this guy away from her. It will only cause problems, mostly for you.
speaking as a boy, i say that you try to take your mind off it by doing something you love and just do that until you like someone else.
If the boy has a girlfriend, find a guy that doesn't have a girlfriend.
just get another boy, the world isn't fair so take it like a woman. There are lotsa cute guys out there!
If he likes you better, then why does he even have a girlfriend?
he may not like you back and ignores you because of that, or he may not understand what you are saying, or he may already have a girlfriend.
You don't if he has a girl let him be and wait ...
If a boy already refers to you as his girlfriend when you are not, and even claims that you have made out when you have not, he is just being overprotective by passing a message to other boys to leave you alone for himself.
do him, and he'll like you (possibly more than his girlfriend).
If a boy has a girlfriend then he is considered taking and you should leave him alone and later he may break up with his girlfriend.
Don't. Simple as that. If he has a girlfriend, he's already made a commitment and like it or not he's with her. You don't want to be a home wrecker. Trust me. Go look for someone else and you'll be happier in the long run.