It could be fleas or allergies, I'd get to the vet for proper diagnosis and treatment. Have you changed his brand of food or anything else recently?
My Dog Has Red Bumps On His Armpits And Thighs, What Can It Be?
This can be many things, allergic reaction or just irritation from some plant, could be heat rash as well. Speak to you vet. He may be able to give you some suggestions to look for and better able to determine what it is