Why would a guy ask to help a girl with homework if she is not his friend?

If he's offering help, it could be that he is simply good at the subject, or just plain likes to help. Clearly, he at least likes her as a friend.

The other possibility is that he's into her, and this is his way of making a move. Less likely, I would think, because if a guy is shy, he will test the waters more before making a bold move like that. And if he's not shy, he would be more direct.