I went out with him on his first ride and I've promised to go out with him for further lessons, as and when we can fit them in. I've already helped him devise a method for getting on and off the bike with his reduced mobility, and I'll be following this thread with interest for ideas on how to help him with gears.
What I've been doing with my beginner ladies is:
meet in the local park, it has some flat paths, some short steep inclines, some longer not so steep one. Ideal.
Then I explained the cogs. Then I got some to hold the bike up, go through the gears while another moves the pedals by hand, to get a feel of "resistance" on different gears.
Then we practiced changing gears on the flat to feel the difference while actually pedaling.
Then we practiced on a few hilly bits, me coaching: chance to small front, change to 3 with your right shifter, and so on.
Alas, the only one that got stuck halfway up the hill was the one on the fancy road bike.