I've not dealt with ASD kids, so I can't directly answer this, but I do have one word of advice. IF you do decide to go ahead with it, may I suggest that one of the first (or perhaps THE first) person that you trade with would be Art, the Greeter, at the Beach Club? He is one of the most magical people at WDW, he's a true living Disney treasure, and without a shadow of a doubt, one of the nicest gentlemen I've ever met in my life.
He does a WONDERFUL job of making everyone feel special. He just about always has both a Yellow (trade with everyone) & a Green (trade just with kids) lanyard on, and is EXTREMELY patient. I have no doubt that he'll be as patient and nonthreatening as you could ever wish for for your son's first trade.
For more information about this magical man, & so you can see a picture of him so you'll recognize him (though, trust me, he will definitely say hi if he sees you & your son, without question), check out this thread:
http://disboards.com/showthread.php?t=1454184 Also, if you don't see Art when you go there, ask at the front desk, they'll let you know if he's around & ask him to come by if he possibly can. Or, call WDW-INFO (or from any on-property hotel, dial 0) and ask them to check if Art, the Greeter at the Beach Club is working, before you even head over. Trust me, it WILL be worth it, even if you don't try pin trading, but just want someone to make you & your son feel special.
Enjoy the Magic, as embodied in Art, the Greeter, or as my wife & I think of him, the Grand Admiral of the Beach Club!