I would be very scared to buy any used bow without a good back ground of knowledge on them, I've been bow hunting for over 20 years and couldnt walk into a shop and just pick up a random 5 year bow and know much about it unless I previously owned something close to it. Someone could pass off junk and you prob wouldn't know the difference.
I would buy him a gift card to a local shop that sells used bows or yes even big box store with a "pro" shop. If he also hasn't bow hunted before he will have as many if not more questions then you will.
You guys could make a day/evening out of it. Let him goto shop learn something and start forming his own opinions and what "feel" he likes in a bow. If you buy something that is wrong for him he may never persue the sport thinking that just how bows feel.
He (I know I would) will prob appreciate the fact you put the time into planning a day kind of just for him. Plus you guys both get to enjoy the time together.
Just getting on here and asking was a really cool thing to do.