Talking Joystick Mouse
I built a joystick mouse for him out of a USB gamepad and arcade machine components. This mouse had two features lacking in the commercial model: 1) It could launch programs or commands, greatly increasing the efficiency of computer use 2) it could talk, giving auditory feedback regarding which button was pressed, as he could not move his head to look at his hands.
Great instructable showing how to modify a games joystick. Note it uses Autohotkey to convert Joystick to Mouse movement. Check out other Joy to mouse software if you dont want to mess with that
We have since used descendants of the first joystick mouse with several people who had cerebral palsy or other conditions that made traditional computer use difficult. The cost to build one of these is about $45.