He is certainly not a happy discus.
What are you feeding him? Beefheart, bloodworms, Blackworms, flakes, bits?
Your ammonia and nitrites should be 0, but not your nitrates. If your nitrates are 0 you aren't using your test right or your filter is broken.
Was he thin when you bought him?
Was he bought with the other discus?
What's the hardness of the your water?
Do you age your tap / use RO?
If its not a water condition or food problem, it could be an internal parasite. Do you have a QT tank with cycled filter?
EDIT: I saw your video on another thread. Is he the one that's swimming around actively and being agressive to the others? If so your pictures on this thread are telling a different story than what you have. From the clamped fins I assummed he was being reclusive and hiding. It still could be a parasite problem, especially if he's no longer acting like he did on the video. I am curious what you are feeding them.