Ask HN: My father had a stroke, know interesting rehabilitation startups?

Another not quite direct answer, but at least in my part of the world these kind of companies are more often "spin-outs" from university research departments and institutes rather than "start-ups" from the general economy.

It might be worth having a look the universities doing research in this area, then seeing if any of them have a group in a business incubator.

Sorry, not a direct answer. You might want to look into Lions Mane mushroom extract. There is a double blind study out of Japan that shows nootropic properties (regrows nerves). Good luck to your father.