I suppose it could be lymphocystis... If that is the case then you need to reduce stress so he can fight it off on his own. There isn't anything available for treating lymphocystis. I would increase water changes to reduce the amount of nitrites in the water, add IAL/tannins, add 1tsp aquarium salt per gal to help prevent secondary bacterial infection... The growth/tumor may shrink on its own...
Personally, I would treat for possible parasites first, just to make sure that isn't the problem. You don't want the medicated pellets since it is
external. You want Jungle's Parasite Clear Fizz tablets or API General Cure.