I would keep them grain free long term. It takes time for immune system to calm down if that is the culprit. Baxter had a rash on his lower belly and penis when I got him. The vet grabbed a bunch of gold bond powder and tossed it on there. A day or three later it is almost gone. Strange because it looks exactly like your picture. And he said it was just heat rash. Maybe they're not the same. Hard to know?