I think that you should stay away from this boy.He has a girlfriend.End of story.
Age is just a number,in fact my boyfriend is 32 and I am 19,I see no problem with that gap while others do.
This guy,no matter how many years apart you guys are,is in a relationship.Do you really think it would be fair to the girlfriend if you two hooked up?
Just imagine if you were in a relationship with a guy and found out that he was hooking up with some other girl.You'd be upset right?
It's times like these that we really need to step outside the situation and walk a mile in someone else's shoes.