Hi!
Go to the fish store, get a ten or twenty gallon tank kit for quarantine (with light, pump, and a heater) and quarantine your puffer (and ALL your other fish too, if you have any) in it and start hypo salinity ASAP!
These kits are cheap enough and keep your puffer in hypo at a specific gravity at 1.008 for AT LEAST ONE MONTH after his spots disappear, then you can GRADUALLY up the salinity to 1.023 to 1.025 (over the course of several days) after that. Keep him there until your ICH die off in your display tank is complete - usually this takes eight weeks!
(DON'T add ANY new fish in there during that time, either!)
Hope this helps!!