Vinegar will likely sting, but it should be safe for him to ingest.

My Sleet has seasonal allergies. She'll start scratching and chewing in June when the snow is all gone and stuff starts to grow. I've given up on all the over the counter remedies. Nothing helped her very much. For the last three years when she starts to scratch, she goes to the vet and gets a shot of Depro-medol. It's very similar to prednisone. For Sleet, Prednisone had some nasty side effects. It turns her into a pyscho puppy. The Depro doesn't work quite as fast (pred stopped the itching within the hour, Depro takes a day), but it has no effect on her personality. Sometimes she needs a second shot in September, depends on the weather. Sleet loves an early frost!