Try moving the receiver around to better locations. Also, if you are using rechargeable batteries, the signal is going to be weaker (rechargeable batteries are 1.2V, rather than 1.5V).

Maybe he could try this one (Amazon.com)

If he works well with the joystick on his chair, perhaps a wireless joystick attached to his chair would work. This one (Amazon.com) looks promising. You can also find some others (Amazon.com)

Do any of these look like they could work? If not, we can concentrate on your current mouse, but I think there's quite a few large hurdles in that effort.