I think he does look kind of like a feather tail but I'm definitely not an expert in that field, lol.
It doesn't look like fin rot to me. The chunks missing from his tail fin could be from him biting his tail due to stress or snagging it on decorations. What kind of decor do you have in your tank?
Also, 100% changes could be stressing him out, as well as the temp being too cool. The salt is probably having negative effects on him, too. I'd remove it and just use stress coat to help his fins heal up.