Start with pinch pots. He can do that in his lap with the least strain on
his back I would have thought.
I like your idea of not making his beginnings too complicated. Even slab
building does take a lot out of you physically as you need to move around
the object a bit to get it all together. If you have a dias/wheel on which
to place the sculpture, so that it moves around instead of him doing so, I
bet that would help.
Personally, just the feel of the clay and the ability to mould it into any
shape I like is wonderful. Even if there is no actual result in the first
couple of tries - just the distraction of concentrating on the clay instead
of the pain may be helpful.
Wishing you the best of luck and him a good recovery!
Marianne