Well, if he's still eating you could try to feed him a specialty flake food laced with a medication for internal parasites. If he's not eating, you could try treating the water of the QT with a medication for external parasites. But again, all of this is just speculation and somewhat haphazard guessing. And if you go this route, you will probably spend 5x more money on medications than you initially spent on the fish, and even after all of that, the odds of survival are probably pretty small. Not trying to be a "downer" but I've been down this road before--spending $40 in medicine to try to save a $5.99 fish and have it end up being ineffective. YMMV, and if you try it, I hope you have success.
But even IF this is indeed some sort of worm in there, and IF you are able to find the right medicine to kill the worm, you are still facing the unhappy situation where your fish is going to have a rather large (but now dead, and soon to be, dead & rotting) worm inside of it; and the fact that there is a rather large area of damaged flesh that the fish may or may not be able to heal from.
Now maybe it's not a worm at all. Could be a tumor. Could be some sort of acute bacterial infection (akin to a human getting a "boil"). Who knows. It's just really hard to say. Now that he's acting more sluggish, maybe that will afford you the opportunity to get a higher-quality picture of the lesion that might help in identifying it.
EDIT: Cardboard should work to cover the tank, yes.
