Are you planning on having the boy actually bring your dog for a walk or just bring the dog out to do his business? If it's a couple of quick, out the door visits, I think that $5/day is fine. Afterall, it wouldn't take him long. It will mean that he will need to be around those hours, though, and will have to plan his day accordingly.
My son is 14yo and in the 9th grade, so about the same age, and I think that he would be happy with $5/day that he could count on. I would make sure that I gave him plenty of notice if he wasn't going to be needed for the day. Sometimes the way a person is treated means as much as the pay! Let us know what he says.
T&B