I thought it looked like a burn, that was pretty deep. How long ago did that happen ? When was the last time the vet saw him ? Be sure to keep any sharp or even rough objects out of the tank that can irritate the thin layer of skin because there's not too much protection there. I would apply raw unpasteurized honey and wrap it with gauze, not TOO tight though. Either that or take all rough objects out of the tank, any log, rock, cave. etc. Then use a rolled up towel for a basking area and apply the honey to his stomach or you can use silver sulfadiazene cream although many studies show that raw honey works best on wounds + burns. That area just doesn't look right and I'd be afraid that it might rip open if something hard jabbed against it.