That's an interesting idea. I had been mulling over pvc but the closed cell my workI would look into some closed cell foam blocks with straps that can extend his leg reach while keeping him "on top" of the kit. That should help while he gets an idea if he wants to pursue drums. If he does, I bet there's some custom orders and alterations in the future.
IF he gets into it and enjoys it and mom and dad decide to proceed to a kit the sonor safari came to mind as one that might fit his need. That's putting the cart before the horse though. First thing is to get him in and start on just a snare to see if he even enjoys it.