A big eye does not mean he is stunted. I regulary use the size of an eye in comparison to the rough size of the discus, and by looking at the eye, it appears the discus is around 2-3".
At first when I saw it, I figured you would have to euthanise it, but upon closer inspection, it just has mild finrot, a probable case of Hexamita and some sort of wound/oulser/fungus on his side.
You can save him, but be ruthless with your water changes and add pimafix, mela fix daily, because water changes will remove most of the meds. Remove carbon and raise temp to 86F. 88 MAYBE.