There's alot of information on HITH. First what are your water parameters? What size tank is he in? What diet is he currently on? What is your water change schedule?
This can be treated with proper diet( no feeders which is a whole other topic) , pristine water conditions with nitrates below 10ppm and if need be you could soak his food in Metro+ by Hikari or Jungles Hole in the Head guard. The last would be a last ditch effort. With proper diet and water conditions he will survive but may have some scarring from the disease.
I rescued a adult pair with HITH and within a few 4-6 weeks there was only one scar as a result of water changes and diet.
I feed only New Life Spectrum jumbo and Hikari Bio Gold and Excel. With the occasional nightcrawlers, blood worms, frozen shrimp,orange slices, peas and beefheart.
With this treatment he'll make a full recovery and live a long 10-15 years.
Good luck.