I would dry lot him and put him on grass hay only. Something in your pasture could be causing the inflamation. I agree with having him tested for allergies.
I would ask your vet about putting him on anti-inflamatories like MSM, Yucca, or something similar. Steroids will cause kidney damage eventually.
You also need to diagnose what's causing the inflamation. If the allergy tests come up blank and pulling him off the pasture doesn't help, then I would take him to an equine hospital that can scope him and run more tests (like additional biopsies of the hind gut, analyze samples of stool, food, bedding, etc.).