A while back, someone came out with a prototype driver for a mouse that used, for lack of a better word, "stabilization" to compensate for a jittery hand. I want to say it was an IBM Alphaworks project, but I'm not sure. Think of image stabilization, but applied to mouse input rather than a camera image.

If a shaky hand is limiting his ability with the mouse, while a long-shot, it might be worth looking into this. (I'll google around a bit and see if I get lucky.)

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EDIT:

The following apparently is only for Windows. Whether you might be able to adapt/adopt it in one way or another, I don't know.

SteadyMouse

http://www.oatsoft.org/Software/steadymouse/

The page mentions an IBM hardware adapter as both inspiration and as a possible alternative, albeit one that (as of 2006) is not yet available.

A comment to the page mentions this hardware adapter. Looks like it sits on the mouse's USB line and is fairly OS independent. I can understand the pricing based on the probable small size of sales and corresponding mode of manufacture. And for someone really needing it, it may be money very well spent. But it's not cheap. (On the other hand, some of use spend as much on ueber-keyboards.)

http://www.montrosesecam.com/index.html

Per the About page, it looks like this is the licensed offshoot/production of the IBM product:

In 2004 IBM secured a global licensing agreement with Montrose Secam Limited to manufacture and market the mouse adapter.