Thinsulate is what they put in winter clothes, etc. Could you make him some? I have no idea how difficult that would be.
We just purchased a 'glove' you wear inside of a regular glove that looks like it has sparkles in the material, but they are suppose to keep your hands extra warm. . . just found the name..... "starcrest of California" they list a Perris, CA address.
Other than that, the only other thing I can think of is those 'hand warmers' you either put into your pockets or inside the gloves
Good luck on your search. I know it's hard. My younger son is 6'6" and has huge hands.
ps: if you want some thinsulate, I'm sure I have some I can share. PM me if you try to make him some.