Try smiling back. Take time to talk to the boy and make conversation with him. You can learn a lot by the way he talks to you and the content of his discussion.
I smile at people that I feel friendly with, even if I do not know them. There is a man I smile at every time I visit a specific store because I think he is funny and when he smiles back I start to wonder if he thinks I am flirting with him even though I walk in holding my husbands hand. LOL.
I guess my point there is that its really difficult to tell what a person wants if they just smile at you. You will have to dig deeper and go in personally.
A question for you is.. do YOU like HIM? If you're not interested, it may be no concern that he is smiling at you a lot and you wouldn't want to get his hopes up by leading him on.
Examine your own feelings and then take the time to get to know him. The other answers above are also really helpful. There is no shame in getting a friend to help you out.
Peace and love forever! xD