Do guys check out other girls while in a relationship???

Wowzer! Does he seem happy with her? Guys usually don't make good friends with girls. They always end up falling for them. He probably likes you as friend, maybe even more, but he doesn't realize it. You seem to think well of his gf, don't get in the way, it may ruin your friendship with this guy. Give it a little bit more time. You can even ask him some indirect question to see how he feels about you and her.