Benadryl
1-2mg per lb every 8 hours
So the dose for a 50lb dog would be 50-100mg
I'm sorry I only just saw this post, and I may be a little late....
Whatever the cause of this it does sound (as seanna said) -like an allergic reaction. Benadryl is what you need. Sorry I can't remember the dosage....I'm going to check it, and will be back in a few minutes.
it sounds like hes allergic to something, anything from bathing shapoo grass, a food.
my girl had this once, the local vet should prescribe a cream after a skin test has been performed... it wont take long and isnt too expensive.
my dog developed a rash on her back a decade ago, went to 12 vets and they where all dumb.
a new vet moved to under my building, put her on steroids for 12 days, and (sorry this might be inappropriate) cleaned her anal ducts.
it did wonders,even some of her hair grew back,no stench. and she sleeps better.
hope this helps.
Its an alle.... I think to food or flea's just feed very bland food like boiled chicken and brown rice in the mean time get an oatmeal shampoo go to your health food shop they might have it there or go on the net to see where you can get it
Even a few flea's can cause this reaction .If no help you have to go to the Vet let us know how you go..