Well, it really does depend to some degree on what he's interested in/good at, etc.
VolunteerMatch is a searchable database of volunteer opportunities. I update information about my agency's mentoring program (they make you update every so often, even if nothing changes, which means that hopefully everything should be current), so I know that one of the things they ask is if it's good for seniors. So I bet you could actually search by that, as well as by location and interests.