Best way to treat goldfish with ich is with salt. You will need to treat him in a hospital setup (you don't want to salt an entire pond.) Gradual (over 24 hr) increase of salt to 0.3% is recommended, and you need to keep the salt at that level for at least 1 week after the ich spots are gone.
Treating Ich with Salt
You can optionally raise the temp to high 70's (26C ish) to hasten the ich lifecycle and shorten treatment. This temp is not high enough to kill ich out-right so salt is still needed. You will need to have excellent aeration to maintain goldfish in higher temp.
For severe infestation, i have also use salt dip to quickly stripe off the slime coat & remove the parasite load. I would use 0.5 - 0.9% salt bath (lower conc. if fish is weak) for 15-30 min. Followed by the long term bath at 0.3%. <Some go as high as 3% - full marine ... but that is tough on the fish.>
During treatment, maintaining perfect water condition is a must. That means setting up a cycled hospital tank, and lots of pwc
. <Some advocate daily 100% water change to remove all cysts that had fallen off the fish.>