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:
So far the controller scheme looks like the RIO uses a joystick to control the radar cursor, so for that any stick like device would do you, like an xbox controller, joypad, whatever.
However, I cannot overstate the sheer amount of mechanical buttons a RIO will have to interface with. Most people have trained on the hog or hornet, which has DDI's, all the functions on the DDI's in the hornet are entirely separate control panels in the tomcat. I would suggest strongly that if someone wants to be a dedicated RIO, it would be a good future investment either making a button box (there's plenty of tutorials) or buy one of the many from amazon, (or this thing from razor), just to have some common controls physically in front of you, (such as the countermeasure thumb stick on the grab bar, and the radar mode selection buttons).