I did read your question! You said that his butt under his tail bleeds after every time he goes to the groomer, and that after he licks it, no blood is visible since.
If your groomer empties his anal glands, that could cause the bleeding. I believe in leaving anal glands alone, if they are normal, and only emptying them if they are infected or impacted.
If his glands are infected, he might need antibiotics.
If the bleeding is coming from his rectum and not his glands, he may have "colitis
", inflammation of the colon. Your veterinarian can check his rectum and glands to see where the blood is coming from.
If this happens only when he goes to the groomer, and every time he goes to the groomer, I might ask the groomer to not empty the glands. If he still is having problems with the bleeding or licking
, I would have your vet check him for infection or impaction.