Is he wealthy? Or is he, well. maybe struggling a bit? If you were in his situation, what would you want someone to charge you? Groceries- people need to shop so if you can't make as much money as you need to, or like to, maybe do your shopping at the same time. Ask him if he would care to join you on an errand or two. He might enjoy the company and you could combine errands to save on gas and time. Even mundane errands can be more enjoyable with a 'friend'. As far as the doctor, bring along a good book or some paperwork to get caught up on too.
I bet there IS a volunteer service that provides the service he needs based without stressing his financial situation...or yours. Maybe do a little phone calls to find such organization or ask some seniors that have trouble getting around because of other disabilities.
But if you do just end up charging him money for the gas, if he can only afford that, remember that you are helping someone in need.