You should first look at his feces with a microscope to confirm that the parasite your green terror has is spironucleus, because if he has another type of parasite like nematodes or tapeworms, then feeding him Hex-Shield or any other type of metronidazole will be completely useless.
Is he still eating other types of food, like frozen bloodworms? If so, you can mix Seachem Metroplex (same active ingredient as Hex-Shield) with bloodworms and Seachem Focus (to bind the medicine to the bloodworms). Or you can make gel food using unflavored Knox gelatin.
If he isn't eating at all, then you can attempt to treat the water with Seachem Metroplex (or any other brand of Metronidazole). Treatment in the water will not be sufficient to cure him completely, but it might reduce the parasite load just enough to get him to eat again so you can feed him Hex-Shield.
If treating the water doesn't work, then you will have to force-feed him.