My elderly dog had Inflammatory Bowel disease. The foods she was worse on were the "hypoallergenic" high quality premium types. She did very well on Butchers/Pedigree Chum. Medication wise she was on prednisolone for life and anti-biotics periodically (though this was more to do with the immunosuppressive qualities of the steroids).

My dog was 14/15 when she was diagnosed and we opted not to go for exploratory surgery either but she did have an ultrasound. She lived for another five years so hopefully yours can have a long life yet!