Is it the big patch of skin missing on his tummy?
If your water is perfect and this is not a result of an attack from a tank mate, then there is a good chance he will heal, albeit you must find out how it came about.
They are literally top survivalists. I find them half dead and missing skin patches every year our seasonal water recedes and they rebound just fine in my tanks.
But again, If I were you, I'd earnestly try to find out how this damage came about. Without this knowledge, it is very hard to say what will happen from here.