I hope hes right handed.

For PC FPS i would try a mouse with a lot of buttons on the left so you could put wasd on 4 buttons on the left so he could control the shooter enirely by mouse.

Depending on how much use he has with his left arm he could also probably build a simple custom Joystick to replace wasd movement keys...

Also i would get some One-Handed Keyboards for fast Typing with one Hand.

Hope he gets adjusted quickly!