Hi Erin! I'm going to have to make this quick but as the mom of 2 boys I've done the scrounging for little goodies and I wanted to give you a few thoughts.
They do have hidden mickey books, maybe even pocket size? I haven't bought as of yet. I did get Robert the Imagineering Field Guides to the different parks - fairly small books, and he
inhaled them. That kind of thing is right up his alley. He's also old enough for his own guidebook, maybe Birnbaum's - it's written very pro-Disney so it's pleasant to read. If he's a reader maybe one of the Kingdom Keepers series? Those are YA fiction set in the MK, I think. Other than that, gum, maybe a Disney baseball cap if he wears caps, sunglasses, snacks (he's probably a bottomless pit by now), some Disney themed music somehow? Some Disney boxers or sleep pants? T-shirts? With the latest Pirates movie coming out soon, you should be able to snag some pretty cool stuff soon.
Oh, just remembered - maybe small
Lego sets, or a starter Vinylmation set? Or a Mr. Potatohead body? Disney gift cards?
Gotta go! Boss is in crisis!

Love everything you've done so far, especially the Mickey head thingies. I asked Robert if he would like to do chores to earn money for WDW, and he looked at me like I had lobsters crawling out of my ears.

"Work" is an abstract concept to him, and he knows his grandmother (my ex-MIL) will give him Disney spending money, especially when we are going around his birthday.