I am not sure about what you can use to make his hair grow in but have you checked to make sure he doesn't have a skin/hair disease? How long has he had the bald spot, and when did you notice it?
good information , I tried olive oil and it helped slightlyI'd definitely want a vet to check this out in case it's a sign of skin disease.
What color is your pup? Certain coat colors are associated with an increase in skin problems that could include hair loss.