Welcome to the world of allergy's for dogs. I have a 7 y/o female boxer (Roxy) who is on allergy shots 3x/week. For the 1st two yrs of her life we fed her WM brand dog food after the Purina Puppy Chow, and she was very healthy but a pooping machine LOL. WM stopped selling their brand when the dog food scare came about several yrs ago, so we started trying different brands and things went downhill from there. She started having red welps or blood blisters, sometimes large pimples all over her body, but did not ever scratch them, they just looked horrible and made some nasty messes.
Finally went to a "dog allergiest", a local vet who specializes in dog allergies, and they did some testing, similar to those for human. Roxy is allergic to grass, cedar, most pollen dust known to man, grains in dog food, etc., so she gets shots of a serum specific for her, just like a human, along with some pills to help fight yeast infections on her skin. She seldom has UTI's, the yeast infections on her skin are one of the causes of the welps/blisters I mentioned. The pills are utilized only when needed but the shots are done every 3 days. We also use some medicated shampoo from the "allergist" but Roxy does not need baths all that much, but she enjoys a good bath whenever she does need one.
We also use a mix of Listerine and water in a spray bottle to spray her feet after we wipe them down with wet wipes after she is walked. She is walked several times a day, so. We also feed her "4Health" brand dog food available at Tractor Supply here in TN, the whitefish and potato flavor. She does not like the Salmon flavor. They have several different flavors but the "allergist" recommended the fish to avoid additional allergic reactions to meat, etc., this brand is grain free and runs about $35-40/ for a ~30lb bag.
If the dogs bedding has cedar shaving inside it, get a foam filled one or try a blanket/sheet, this one move helped a tremendous amount for Roxy as sleeping/laying on it so much just defeated a lot of our efforts. I even keep Roxy away from the 4 or 5 cedar trees the line our property lines. She does have flair ups, but with the meds, she recovers much quicker, its not cheap, but Roxy is a member of our family, so she gets some pretty good medical care when needed.
Google vets specializing in allergies or similar and see if there is one close to you. It's not cheap, but it could end or reduce your dogs suffering, and yours as well. GL and hope your boxer gets better.