It’s probably going to be a lot of trial and error. Has he ever tried a track ball mouse like this: https://www.amazon.com/gp/aw/d/B01936N73I/?_encoding=UTF8&pd_rd_plhdr=t&aaxitk=de428092487f481b8a58e3baf9d66528&hsa_cr_id=7322648830001&ref_=sbx_be_s_sparkle_scm_asin_0_img&pd_rd_w=FK1JL&pf_rd_p=22aaccd3-32f1-4e8a-8b82-495782857be5&pd_rd_wg=HZpni&pf_rd_r=XZGAXP7PH240TTA588BP&pd_rd_r=349e2075-238a-4829-8614-981a2ee24239
If you can, I’d also suggest using voice control tech. Apple devices have voice control accessibility features.
For keypads you could try something like this:
Sorry if I'm just completely missing it, but when I look at the pictures here, it looks like the trackball is completely flat, not at all tilted:
https://www.amazon.com/Kensington-Expert-Wireless-Trackball-K72359WW/dp/B01936N73I?th=1
Tread Labs Insoles. Way better than the kind you get from the Dr. Scholl's machine at Walmart. They have different arch heights and firmness so you can get what works for your feet, and they're really great about returns. I use the blue ones!
Trackball Mouse. This thing has been life-changing for me. I tried a vertical mouse prior to this and while it helped my wrist and hand problems, I was still in a lot of pain. I don't understand why it's so much easier on me than a regular mouse, but it works. I would recommend an actual trackball like the one I linked over a standard mouse with an added trackball.
If his left hand is at or near 100%, I'd definitely recommend a trackball. It doesn't have to be that toy-like one, you can get relatively large trackballs that look professional: https://www.amazon.com/Kensington-Expert-Wireless-Trackball-K72359WW/dp/B01936N73I/ref=lp_172499_1_3?s=pc&ie=UTF8&qid=1542739066&sr=1-3
I've found trackballs are relatively easy to use with your off-hand, so hopefully he won't have too much trouble adapting to it.