I've had a good trawl round the internet (I've got an oscar too and am petrified of him getting HITH) and first of all you need to improve your water with more, larger water changes and the fish's diet with a more varied menu; soaking the fish's pellets in a liquid multi-vitamin (one that contains vitamins A and D) seems to be recommended by a lot of people.
Then you need either Flagyl (that's the brand name, the drug is metronidazole) or a mix of Flagyl and Chloramphenicol (chloromycetin). You might need to speak to a vet and get a prescription, depending on where you are, because chloromycetin is an antibiotic.
I hope it turns out to be something else; I'll be keeping my fingers crossed for him.