Like most have said, just say thanks next time you see him, and perhaps mention that you didn't expect payment. Offer to help him out another time- this time without compensation!
Oh, and Mooseman- if you are qualified (as in, you know what you're doing), you deserve how much you're worth, even if you are a "kid"! nwlinkvxd makes 26$/hour as a computer programmer and he's 20 years old and still in college!
/me insanely jealous
