
My American Cocker Jesse suffered from the same, though it wasn't on his back, but on his chest, belly and legs. It was a skin infection that cleared up with antibiotics. However, the infection came back and when it kept reoccuring a couple more times over a year, we decided to dig deeper. He was allergy tested and came up with lots of food and environmental allergies. Adjusting the diet helped, but didn't solve the problem entirely.
Due to other symptoms (mainly also behavioural), my vet and I agreed to do a thyroid test eventhough Jesse was less than two years old and Jesse is indeed hypothyroid and being treated for it. Touch wood, since being on thyroxine for the last two months, he has not had any further infections (though is still a bit itching on and off, but skin problems take a long time to rectify after thyroid treatment started).
So I think to start off with, your dog might well need antibiotics and I would also adjust his diet to something natural if he isn't already on such a food. If the problem persists or reoccurs, then further tests may be advisable.
Also, how often do you bath your ACS? I have found than an aloe vera and oatmeal shampoo is very soothing on Jesse's skin and seems to help.
Vera