Hi Tom,
A picture would help here, but this sounds like its not HITH from a parasite (a good thing) Sometimes the pits in the fishes head get infected, and rupture (think pimple) It leaves a hole. That should close up and will not leave a scar unless its massive and doesn't heal well. One thing you can do here is net the fish and swab the hole with hydrogen peroxide or iodine. Lots of clean water and the usual care for discus here should help. Since you have started metro... continue that for 10 days at your current dosing.
hth,
al