Does anyone have experience with a dog having painful pooping and blood in the stool?

My bull terrier who will be 7 this year has, for 3 years now, had bloody stools with diarrhea and just recently apparently painful bowel movements (he cries out in pain). We have taken him straight to the vet where they checked him when symptoms first started and said rectal exam was normal but confirmed blood in his stool.

They wanted us to follow up but we moved across the country (hubs is military). We followed up again at our new location and were referred to a local vet for tests and imaging.

Long story short they ruled out everything including prostate, anal sac issues, and his ultrasound was clear. No constipation or inflammation. They put him on antibiotics thinking it was a colitis. No improvement with antibiotics either.

At this point the vet is stumped and so am I. I am wondering if it’s a food allergy? We have another dog with zero problems in the same exact environment. Just throwing all this out here for anyone who may have had a similar experience