if he is totally rid of fin and tail rot it should grow back. One of my hybrid catfish had half his tail fain ripped off in some rocks and it grew back in about 2 months. Also I have had a marble catfish with fin and tail rot in the past and after it healed he had no fins and no whiskers. They all grew back in about 3 months, except his caudal fin(tailfin) it had rotted to where the bone behind the fin rays fell out. So to answer your questions if the fin rays still have at least one section they should grow back. Make sure that you continue treating him for a while even if you think he is healed.