OK -
How about this possibility:
He is happily involved with his girlfriend but happens to find you attractive - and you caught him looking at you.
That can happen
Just because guys have girlfriends doesn't mean that they don't find other women attractive.
He might just be embarassed that you saw him looking, and now doubly embarrassed because he has your attention.
But just because he find somebody attractive does not mean he is not happy in his own relationship.
If he says he has a girlfriend and makes no obvious advances towards you - I personally would accept that at face value and move on.
