there are analog sticks that have a rotation. I believe these guys have them.

P-Q Controls | Heavy Duty Industrial Joysticks and Foot Pedals, Hall Effect

but if you look at the guts of a joystick it usually looks like a cube. it might be very easy to mount a rotary pot to the cube and modify an off the shelf. for that matter you might be able to take a standard stick and mount a rotary pot (knob) on the stick. like a clown nose at the end you grab.

but if he can use a foot you could also setup a throttle pedal or chin pedal or armpit pedal.