Have you tried Prescription I/D yet? Especially if Hill's is already working better than some other foods. If he's having all this digestive trouble it might help at least in the short term. It proved so wonderful for Beth after a bout of pancreatic inflammation I've kept her on it as her regular food for over a year now (kibble with some canned on top) -- finally no more poop issues, her coat is like silk, etc. There are other dogs here on GT who need to be on it long-term. Or Science Diet Sensitive Stomach is sort of like a non-prescription version though it is not the same. I'm not saying your dog might not have a medical issue, but I thought I'd mention it.
I am also one of my vet's two best clients, though at present that is based on a cat with serious issues as well as my greyhound and her various issues and injuries (she's healthy over all though!).