I had a 3 year experience as computer graphic teacher from 1998 to 2001 with a mixed classroom of people with various handicaps. At that time we made a lot of webdesign and some 3d with Bryce 3D.
Each man or woman has its own difficulties, there is not a general rule, but:
I noticed that some tethraplegic students could work better with a trackball.
In some cases we altered the mouse so that they could grasp it better; it is quite easy and cheap. If your man can not press well the mouse buttons, you can glue over them a small table-tennis (ping pong) ball. You also need a couple of rubber rings to put between the balls and the mouse buttons.
A programable keyboboard for gamers, like the Belkin Nostromo, may work well beacuse you can split some functions over two hands: move the pointer with one hand and control the mouse buttons and wheel with the other.
In the worst cases, we also used the Dragon Naturally Speaking software as help to input voice commands.
Sorry for my bad English, I hope I didn't use rough or rude terms.