I mean it looks better than it did when he was out of the tank. I was reading up and trying to find my own information, since people seem to lack consistent ways of dealing with shell rot, fungus, etc etc. betadine apparently regardless of it's diluted or not, still works and people give what's called a "weak tea solution" I've actually seen NERD (reptile keepers) use this when they got their shipment of turtles in. That's used more for a generalized entire turtle preventative, or if they have damage from shipping. What i've seen for cases like yours; is they apply the Betadine and let it sit for 10-30 minutes, wash it off then apply something called Silverdine cream which you can only get from a vet ( I tried looking online everywhere ) and let it absorb which takes about 30 minutes, then pat it, make sure the cream is absorbed and not still on the turtle, and put it back into the tank. Idk how I feel about the weak tea solution as I'd rather not have my turtle ingesting the Watered down Betadine, but that isn't needed in your case.