Why has he gone really shy with me now he knows I like him?

Im like that. I get all nervous and shy if i like someone. Maybe it's because you dont know if the other person likes you too, or if they arent in a relationship already, or if it's ok to feel the way you do. I think you should just confront him if you like him. That way you get an absolute response instead or conflicting thoughts of whether or not he likes you too. So if necessary you can move on.