I think pin trading could be a great idea for him!

It might be good to try and prepare him before you go, with either social stories or role play. Pin trading is a very easy thing to break down into steps (please excuse the smilies, I got carried away

):

Introduce yourself to the CM

Ask to look at their pin lanyard

See if they have any pins you want

Point to the pin

Choose which pin of yours you want to trade

Swap one for the other

Thank them for trading
A few quick questions:
Would he be all right with giving away one of his pins to get another one, or might he get too attached to his pins and not want to give them up? If he might get attached, it could be easier on him if all of his trading pins were the same, so he wasn't giving away something unique and precious
Would he be OK with the idea that he didn't
have to trade every time, if they didn't have a pin he wanted? This could be something you work on in the social story / role playing, so that he knows it is OK to say "no thank you"
Would he be able to cope if another Guest came up to him to see his pins? If not, you could always keep his pins out of sight, and just bring them out when he wants to trade.
Would he be all right with not being able to buy every pin in the store that he liked? I don't know whether this would be an issue with him or not, but if it would, might a limit or 'daily allowance' help him?
Also, very random thought here, but is he OK with having things around his neck? If not, a pin lanyard probably wouldn't be the best way to go. There are plenty of other ways to display pins, including putting them on a hatband, bag, sash, strap (for instance the straps you get on those battery powered fans) etc. It would be best not to put them straight on his shirt, as the backs of the pins might get uncomfortable, especially with some ride restraints (such as Stitch), and would make it hard to take them off for trading.