relax.
HITH is easily reversable, but you wont see results overnight. so dont expect to start treatment and he'll be all better in a week.
it takes a few weeks for hith to completely clear.
Just get a siphon filter from petco. If you never got one to start with, thats most likely the cause. The gravel has been accumulating way too much poo and food and your ammonia probably is up as well as your nitrites.
If you dont have a test kit, get one. They are imperative to fish keeping.
Test your water and make sure your levels are as follows.
Nitrite - 0, Nitrate - less than 40 ppm, Ammonia - 0
Remove any carbon you have from your filters. Carbon is pointless anyways. It masks odors that would otherwise let you know that your tank is in need of maintenance.
Salt will help. Salt at 1 Tbsp/5 Gallons
(unless you have plecos or other scaleless fish in there too and make it 1 Tbsp/10 gallons)
Also, you can purchase some Malefix. It is a natural healing aid with Tea Tree Oil that heals open soars very well. Find it at the LFS.
The Cichlid staple is fine. But just pellets only is not ideal. Check the ingredients also. Make sure the main ingredient isnt corn or gluten meal. In fact i dont recommend any fish food that has corn or gluten meal as an ingredient at all. It is difficult to digest and can cause blockages.
He needs a more varied diet. Give him crickets, worms, veggies like carrots and peas, shrimp, white fish.... have fun with his food. Oscars in the wild eat bugs and anything that passes by. An Oscar's diet should be varied.
And of course water changes. 50% every day for a week. Then you can switch to 50% every 3 or 4 days after that.
Keep track of your parameters.
HITH will heal in time if you do all that. But expect the holes to take a few weeks to go away completely.
Some cases of HITH that are extremely bad can take months to heal completely.
But you'll notice your fish will seem to be a lot happier.