How long has he been on the pred? How are the fistulas responding to the pred? How old is he and how is his general health? I think it is better for you to put him on a single novel protein diet. The Natural Balance line is really good for this. Something like Fish and Potato or Venison and Potato. We are trying to find a food that will not trigger his immune system and cause an adverse GI reaction.
I would look into the cyclosporine and ketoconazole treatment plan. Ask your vet about it. This course of treatment has the highest chance of recovery and remission of PF. It is a little expensive, but the keto helps the cyclo absorb in the system better so that really helps with cost. Sam's Club and Walgreens have good deals on these medications. Surgery, cryotherapy, tail amputation etc are no longer recommended. Keep the area clean and hair free. Clip the area around his anus and under the tail. Try using Desitin on the area. That will help. I also like to keep the area clean with Vetericyn.
Try going to this site and joining the yahoo message group. The formatting of yahoo groups is pretty atrocious, but the information is very good.
PF-L : PF-L
Keep your chin up. It's an expensive and terrible disease, but we can manage it with medication, diet and different therapies. Some others will come along to offer "alternative" remedies so that's an option as well.