huh that sounds like a pretty nasty case of parasites. I’ve never dealt with that before but i’ll post it in case anyone else has.
Whatever that is, he needs an anti-parasite medication, pronto. One of the meds that goes in the entire tank and is meant to treat internal worms ought to do the trick. If the freshwater dip made some of the parasites come off, that’s probably worth repeating daily. If not, don’t.