I love him but he has a girlfriend?

I cannot emphasize that when someone has a girlfriend or boyfriend, that person should not interfere with that relationship and leave it be. You have to remember that a lot of drama will continue to work within that relationship and it will cause a lot of fighting, unnecessary drama between them and probably even cause them to break-up and not even be friends afterwards.

You do not take that step in telling him that you like him (at least wait a couple of weeks should the relationship end between him and his girlfriend or when you think he's ready for another one) because then he'll have mixed feelings perhaps (after you spend enough time with him and he gets this fixed feeling on you, and that will only cause problems).

You have to remember that when someone spends enough time, they're bound to like each other despite their differences or whatever causes them to dislike each other in the first place (however, I assume you two are friends anyway but I'm not sure of that). Just how most of the time it works and people don't even realize it because emotions can be a very "roller-coaster" like thing in life.

Remember as I said do not interfere with the relationship as it'll cause drama, the best thing to do is become friends with this person, learn more about them (this is if of course that you two aren't friends as it is) and afterwards if the relationship between him and her don't work out, and after waiting a couple of days or weeks then go to him, make sure that you can make a mark towards him.