How long has he had it? It actually sounds kinda like he got into some bull nettle, but it could be anything. Did you take him anywhere different or unusual? Be careful of what you put on it, as he will probably lick it off. If you have any panalog try that. It's an ointment like Neosporin for dogs that won't make them sick if they lick it off. Cool (not cold or ice) compresses usually help a rash. It's probably burning from the licking. They start licking cause it itches then the licking makes it burn. Try to keep the cool compresses (cool damp cloths work best) on it and try to keep him from licking it. You can also give him Benadryl if you have it. Give him probably only one capsule. Let us know how he's doing.