DD was five on her first trip and really got into the pins. However, she didn't want to trade any of her pins away because she didn't have doubles. LOL! (The Easter Bunny got her goat, though, and surprised her with 15 pins from Ebay that were all the same one. Now she has to trade on our next trip. LOL!)
I would suggest figuring out an amount and letting him buy a few pins. However, don't be surprised if he doesn't want to trade them away, kwim? He'd probably be happy just having a lanyard with a pin or two on it...