There is medication out there, that can regrow hair and turn grey hair back into its natural colour.
I know because it happened to my grand father, but the problem is, that kind of medication is not for everyone, its used for cancer patients and suppress immune system and so on. Also it haven't been documented to work for hair loss.
So the medication is out there, but its likely never going to be for hair loss anytime soon if ever.
He was probably using a JAK inhibitor, there are actual studies going on with these kind of drugs for hair loss, primarily for alopecia areata however