What was he thinking acting so awkward and stalkerish?

Are you asking why he stopped looking at you, or why he did so in the first place?

It isn't uncommon for someone to look at the person they like, fantasize, and just be dreamy about it. Guys do it by staring, girls do it by writing and talking with friends.

You also have to be careful when calling something stalkerish because what he's done isn't follow you around but just look at/notice you when you were around. If he was stalking you he'd be following you. This guy just sounds like a love torn, shy guy who never got the chance with the girl he liked.