If a boy has a girlfriend can he still like someone else?

Yes, people's feelings change all the time. The boy should at least be honest with himself and the girlfriend if he doesnt think the relationship is what he imagined. He should tell the girl and move on with his head held high for being mature enough to not do anything prior to breaking it off.
I say you shouldn't ask him out because his relationship with Her can break down so you will have to wait. When the time is right when they have been broken up for months and he is single. You can make a move on him and get to know him better too. One of you will ask each other out just don't let the friendship end. If one of you say no, then just stay friends. If one of you say yes, go for it.