I recon it would be easy to make him a custom controller to game with if someone wanted to. Having a quick think, I came up with something like this:
Bare in mind it's very bad, just an idea to use roughly.
The mouse movement would be a trackpad with a stick attached with tactile feedback, basically when you move the stick in a direction, it will try and center itself back to it's original place. What this does is basically acts as a mouse movement for him to look around, and when he took his chin off, it will center itself back so it's easy to aim again after.
Above that on a slope is a button array, which are easily accessible, the thought was that he could push the buttons using his tongue behind his lip, thus he could aim with his chin and shoot at the same time, along in that array would be the typical gaming buttons, like "Use" "W" and some spares which would fit around everything which could be customized like a normal keypad.
Of course another idea would be to put 2 buttons on top of the stick, which could act as left and right clicks, overall I think something like that would provide much better control and would probably make his gaming experience better.