Tell him straight out that you think it's more appropriate that he looks among girls his own age, and you will not be dating him.

You don't have to stop talking to him, but do distance yourself just to be on the safe side - make sure you're not acting in a way that gives him hope. Don't be over affectionate, familiar or friendly. Be jokey, kind and polite of course. You don't have to treat him coldly or badly. Just don't get too close, don't talk to him outside of work, or too much about your personal lives etc. 

I think you're overthinking is and putting too much importance on what is a little crush on his part. He is a kid with a crush, not a scary monster. A 'no' would make things temporarily awkward but if you still act kindly around him, he'll get over it quite fast. Teens tend to do that.