The only way to diagnosis conclusively IBD is through biopsy which after what your dog has been through, I wouldn't do.
Beau has "severe" IBD and after months of different foods, prescription drugs, OTC, herbals, x-rays, sonos, etc., after biopsy we started him on pred and imuran. The food is z/d which is hypoallergenic dry, wet and cooked tilapia. He gets nothing and I mean nothing else. anything else we give him would put him back into a flare.