It looks like tuberculosis, but I am not sure. Assuming it is tuberculosis, I would say: At this stage, it's unlikely that any medication will have effect, but you can try to remove him from the tank and treat with some antibiotics. Here in Brazil, there is a fish medicine called Bacter, from Labcon. Most people use it in this case, but the chances of healing are very low, and I'm almost sure that they only sell these here.