Could be some kind of allergic reaction but unsure has anything changed? like the diet? what is the dog being fed? what season is it there? can be enviromental, food allergy, or something different also when did this start?
Is it sensitive to the touch? Does he itch it a lot? It LOOKS (as far as I can see) like an allergic reaction. get some steroid cream from a drug store (it's over the counter) and put it on it. If it seems to irritate it more just wash it off and dry him good. Try using a baby shampoo/body wash on him too. He might have sensitive skin. I hope I helped some