If a guy looks at you and puts his head down or smiles to himself when he sees you does it mean that he likes you?

I would see this as a definite sign that he likes you, but it may be that he's shy. Next time you see him, go up to him and start talking to him. Some good questions are ones like "how are you," "what are you up to this weekend," etc. Just make yourself look like a casual friend because if it looks like you like him and it turns out that he doesn't like you in "that way," then things could turn confusing. My advice is to stay as a friend until there's a definite yes or no as to whether he like you or not.