You need to find out if the guy loves you or his girlfriend. You can then make a decision based on his response. Alternatively, you can just let them be and look for someone else who is not committed and reciprocate your love.
Respect the fact that he is in a relationship and stay away. How do you know he likes you? He shouldn't be broadcasting something like that if he's already in a relationship. He sounds like just a crush, and you'll be over him in no time.