Do you think he could work with a very simple cause and effect switch?
One of my favorite switches is the bead curtain one. Basically it's like those hippy bead curtains, and if you move the beads it activates something electronic. I wonder if you could put it on his wheelchair tray, and put his good right hand through the beads. Then hook it up to either something that makes music, or something that vibrates, whichever he likes better. If he moves his hand at all, the music or vibration will start for a minute and then stop. If he wants more, he needs to move again.
If the bead curtain isn't good for him, you can find other ones that are operated in other ways, by leaning his head to the side, or by touching a button. Lots of choices.
I think for a child with little control of their environment, learning to operate a switch can be a powerful thing.