I've got a couple Henry's my father left me. One is brand new, never has been shot. That one is a .22. I vote for a Henry if your gonna do a rifle......Need to find what is most familiar to him since he is aging.
I would also say pump shotgun, but, as we know, practice makes perfect. If he's not gonna practice much, the need to find what is familiar to him becomes much more important.
Losing dexterity/fine motor control in his hands, I'm sure he can rack the slide on a semi-auto. Maybe try a 19. I think the size is perfect. Big enough to rack the slide for arthritic hands, and shoots soft enough to not kick those hard, brittle bones in his wrist.