If he says he likes you and from his attitude. If they are happy together I would let them be unless he would rather be with you. You can still like a guy and not be his "girlfriend."
dude, if you feel that your girlfriend is being distant, or she all of a sudden doesn't want to hang out with you, or just lost communication between the two of you, you are defintely being cheated...
Just talk to him about it and see if he feels the same way about you, if he does, then he may break up with his girlfriend for you. If he doesn't feel the same way then maybe you should try moving on...
Ask him. Or check his myspace, facebook, bebo, msn profile, anything like that. You could also ask his friends. If you just are starting to like him and you find out he has a girlfriend, steer clear...