Pop Eye itself is not contagious. However whatever caused it may be. Pop eye can be caused by numerous things. It can be a bacterial infection, it can be caused by a scrape or a bruise. It can be caused by bad water conditions. You said your water levels were ok, so i am guessing he scraped it and then caught a bacterial infection. That happened to one of my Bala sharks awhile back and all I used was Melafix. It cleared up within a few days. So my recommendation would be to try turning up the temperature a notch or two, and then adding the recommended dosage of melafix for your size tank.
Unfortunately medications which can be used with cories are limited due to them being scaleless. So defiantely try the melafix since it is all natural. I wouldn't leave the water untreated though.