Why would he look at me a lot when he already has a girlfriend?

Pfft, you are not stupid and don't be too disappointed. He stared because he found you attractive, which is a good thing. You made a move, good thing. He was straight up and honest and told you he was seeing something, very good thing (honest). Now be friends and just be you. You planted a seed, and if she's ever gone, bet you 10 bucks that seed will grow. Never, ever be sorry or feel stupid that you told someone you liked them, it's a compliement. A big one!