How much should I "pay" an amateur photographer for taking wedding pics?

He is just an acquaintance and a teenager and last minute has said he can take pics for my wedding which is a life-saver because I couldn’t afford a pro. So don’t tell me I need a pro, because I want an amateur. However, I think it would be nice to give him some money even though he didn’t ask for any. How much $ should I give him?
By the way, I myself only make like /hour.