My pup had a little #2 accident 2 days after spaying and decided to roll around and play in it (yeah, it wasn't pretty). I took her in the showers and give her a quick wash. Soap and water won't cause infections, it's the opening up of the stitches that vets worry about when they give you that whole list of things not to do. I took a sterile gauze and dried up around the incision really carefully afterwards and she had no ill-effects whatsoever!
Of course, your mileage might vary, but if you dog is scratching around the incision because of the allergy, I think you risk opening up the stitches by now cleaning him. Just my humble opinion.