I would recommend 1 cup of ground flax per day, for 1 week. If the hives start to go away, then leave him at 1 cup per day. If after a week, he still has hives, do 1 cup of ground flax, twice a day. When they go away, then try to go back to 1 cup a day. Flax is great for allergies, hives and inflammation.
This has worked great for a friends horse that got hives, everytime it rained. The vet tried everything, changing food, steroids and such. I told her to do the above and now the horse is hive free, on 1 cup a day. She ran out of flax and did not use it for 2 weeks. He broke out in hives and she has not run out since.