I believe that it could possibly have to do with his past. When someone has a tough childhood they tend to either rebel and not care about what others think, like a bad boy (fighting, drugs etc.) or they will close themselves off and focus only on interests.
In both cases the individual closes themselves off from letting someone in, but they show it differently. They bad boy individual will just be mean or emotionless to you while the closed off individual will appear extremely shy and quiet and keep to themselves.
If this is the case for him, then he might be fighting with himself. It took awhile for him to even open up a little as friends and now that he knows you like him he is fighting with himself on whether to let you in or not. Simpler terms, his heart says he likes you but his mind is telling him that he will get hurt if he lets you in.
