Have him do the moves seated with his feet on the ground.  Take a look at this description I wrote to get you started..there are some special directions regarding sitting position.

He can then alternate between sitting and standing, and gradually stand more as he gains strength.

http://knol.google.com/k/how-to-do-tai-chi-at-home#

This way he can follow the arm movements of your Video and get his chi flowing.

Be sure to ask his doctor if this type of activity is good for him.

Good luck!