Heya Skeeter...

I work as a carer in the intellectual disabilities field, and I "play dad" to a friend's 5yo son (who is currently sitting in my lounge playing with my PS3) who has a condition similar to your boy's, so I can understand the extra difficulties, as well as his need to be taught by you rather than a stranger. My advice would be to talk to an instructor and explain the situation, and get some tips on teaching methods. As Red Head suggested, getting an instructor to set up and teach you to use the buddy box would also be a great idea. If your wallet can stretch that far a PC simulator can also be a great training aid or for use on windy/rainy days, and a decent one can probably also be set up to use the buddy box.

Once he's comfortable with flying you can extend the relationship buy learning to build planes together... this is also a great tool for helping him with his patience and concentration, hand/eye coordination, fine motor skills etc. I'm looking forward to when my friend's son is old enough for me to be able to teach him to fly too (plus it'd give me an excuse to rebuild my old trainer).

Good luck to both of you