A few tips we've picked up over the past several trips. My son is also 10 at the moment but he's been trading in some fashion since he was around 6.
First off go to one of the shops and pick up one of the cheaper "starter" packs. You can use those to trade Disney Cast Members while waiting in lines or any other place really. Most are more than happy to trade, even the cheaper pins, so it's a good way to trade up over time.
If he gets one that he really doesn't want to trade then he can turn it around backwards so it's not visible.
We've always used pin trading as a way to make our son more assertive, we rarely do the trading for him and instead use it as a lesson for going and asking for something he wants. That way he knows if he really wants to get that awesome Goofy Basketball pin from the cast member he's gotta be the one to approach them and ask about it, hopefully setting him up for similar battles as an adult
One issue we had was backs falling off, so if you have some really valuable pins or ones you just know he'd be sad about losing get some locking backs to keep them locked tight. I've had to search a large portion of The Studios for a Jedi Mickey, ended up just buying a new one to keep him happy
Most of all make sure its a FUN activity and not a drag. A lot of times my son will be gung ho about trading for the first couple of days then it dies off and we start leaving the lanyards in the room. One other thing we like todo is make him earn the money he spends on pins that he purchases, they can add up. He always earns money during the months before our trips and then thats his spending cash while we're in the parks.
Have fun!