Try to get some pictures of it. I'm assuming it's not infected since vets have seen it, but the way you describe them that might not be safe to assume.
Does it seem moist or fresh? Is it "weepy"? Is it dried out looking? Katie is right, make sure there is nothing in his cage that he is injuring himself on. It wouldn't hurt to dab a tiny bit of neosporin on it either. That will help seal it from dirt and germs, and help it keep from getting infected. Some snakes are slow to regenerate when they are hurt, and this is especially true if the wound is deep or their bodies are busy fighting germs at the site of the injury.
Double check your temps and make sure he's got a nice toasty place to hide, as well as a place to cool off. When reptiles get sick or injured they need access to a nice warm safe place to heal well, and it gives their immune system a temporary boost when they are sick. Usually raising high temp zone by 2-4 degrees works to fill that need. Just make sure he has that cool zone to escape to if he wants.
Katie's suggestion to change the substrate to paper towels is also very good, I would do that if I was you. That way you can clean it really easily, and no poop will get missed and no debris will get stuck in his owie.
Good luck getting him fixed up, let us know how he does!