Hello! I have a couple of friends that I've helped out over the years finding the best solutions for their disabilities. One friend has M.S. and another had a stroke and is confined to a wheelchair also with limited use of his arms.
anyway, I have a couple of ideas for you...
first off is one of those driving controllers...similar to this one:
http://www.gamecritics.com/review/hhracwheel_hw/main.php(there's a few different styles)
or maybe the "eye-toy"
http://www.gamespot.com/ps2/action/eyetoy/preview_6076428.htmlthe fishing controller may be of use to you, as well as the light-gun, as they don't use many buttons.
But the best of everything may be an arcade stick, have you tried that? You wouldn't have to worry about holding it, you could just put it in your lap.
Just some things to think about, I wish you luck, I just know you'll find something that you really like!

-BobBorakovitz