A photo of the area would be really helpful.
Ich is tiny, salt-like specks which normally cover the whole fish. It's also associated with flashing (rubbing on substrate or decor) and heavy breathing. With a Cory, you'd probably notice the breathing even more than on a normal fish, because he's coming up to the surface for air. Have you seen any of these specific symptoms?
HITH (Hole-In-The-Head) is generally associated with Cichlids, Saltwater Angels and Tangs, and others, but I've never, ever heard of it on a Cory, or, now that I can think of it, on any catfish. It usually shows up as small "pits" that get larger over time, with some whiteness around the area. Does it look something like what I'm describing? What shape is it? Does it look "deep" or "shallow?"
Parasites are very easy to rule out. Have you added any new fish into the tank recently (within the past month or so) without quarantine?
Does this look like something that has developed as the result of a scrape or a scratch? Corys have pretty good armor, but I suppose it's possible that he could have gotten a little bump on the noggin. Is it furry or fuzzy at all, like fungus? Or possibly red at all, which would indicate bacterial infection?
Could you please list all tankmates so we can get a good idea of how possible it is that he's gotten into a fight with someone? Cories aren't really great with the whole territory thing -- in fact, when placed with aggressive, territorial fish, this is often a big problem. They just don't get it, and so they continue to enter the fish's territory until they finally get beaten up or killed.
Also, if you could provide the most recent ammonia, nitrite, and nitrate readings, it would be a big help. Water quality is linked with the possibility of bacterial infections and fungus that would grow on an open wound, so even if you don't think these have developed yet, it's always nice to have peace of mind that you're keeping water as pristine as possible to keep the fish's immune system good and strong
I know there are a lot of questions here, but I think that answering these will get us further toward figuring out what's going on.
