I know how frustrating it can be to try to figure out what's causing the hives. It's a trying, tiring process of elimination. We tried everything, washing his fleeces in natural hypo-allergenic soap, changing his food, since there are a lot of food items that Boxers are allergic to. For a while, we thought it was a red apple Christmas tree ornament that he had chewed up that left red dye spots all over our white carpet. Nope, not that either. Finally, I noticed the hives came mostly at night, and would kind of go away during the day. We came to a tentative conclusion that he might be sensitive to cold at night, since we left the furnace off and this was January. We began tucking him in his bed with a cover every night. I also started putting doggie T-shirts on Scrapper to keep him warm. He wore them for over 2 months. Either they kept his fur away from whatever was giving him the hives, or they kept him warm, because the hives finally disappeared. I also kept up the benedryl, everyday. Occasionally, I would use the cortizone, with the next day peeing problem always appearing. I hated that. I also changed his food to Avoderm Lamb and Rice which has no corn, no soy, no other items that tend to cause allergies. There is also Natural Balance duck and potato, also supposed to be very good for allergies. Finally, I think we have them under control. But, I still notice, when he goes to the dog park, he gets a few hives down his back due to the other dogs' saliva. They go away in a couple of hours.
I sympathize with you completely. It's a long, tiring process to discover what's causing the problem. Boxers are just so tender and sensitive despite their size. Sorry to go on so long. There are also canine allergist DVMs who will do tests to try to determine what the dog is allergic to. I had made the appointment for one, but the hives were getting better by then, so I cancelled it. Good luck.