Stop being nice! I think the going rate is $15 to $20 per half hour. I would not expect someone else to clean up if my doggies had an "accident." The litter box, yes, if I were out of town, but that is it! If my dogs had a poopy problem, I would be concerned and would not expect someone else to deal with it! I would get them to a vet!
He is taking advantage of you. Friend or neighbor, it is what it is! I would be curious to know "why" the dog has potty issues- it may be something besides old age. New food or diet, bad teeth, virus, parasites- he should be seen by a vet and I would recommend that to him before I went back there. Are you disinfecting your shoes and hands before going into another person's home?
It is hard to be confrontational but it sounds like you need to have a candid talk with him!