Ahh the little Blue Cobalts/Diamonds always seem to look really ropey and depressed and odd colours for ages when they are young, I always found they took longer to mature into nice colours than any other discus!
That said.... two things... no three things....
1- Check your water stats! Doesn't matter if the other fish are fine, young discus are more sensitive than older ones and if one isn't happy... there is a reason.
2- Worm the discus!! It is important to worm discus at least once/twice a year, especially when young. Kusuri Discus Wormer 5g will set you back all of about £7 on ebay and will treat the tank several times over. It may save the fish.
3- Check the fish's diet, I know some of the big discus breeders really push beefheart as a good diet and really, it isn't all that ideal. Make sure the fish is getting a good mixed diet of frozen foods (bloodworm, krill, mysis, brineshrimp, discus mix and a little beefheart) and a good quality sinking pellet like Prima or Novo Discus etc.
And I suppose it goes along with checking water quality but had a fair few discus keepers recently at work who's KH had crashed and the PH had dropped. Am not sure of exact reason the the RO water (I am assuming you are using RO!) is unstable and if the KH drops, the PH does, not sure if temperature affects it as only thing my customers had in common was high temps!
TMC do an dH/KH buffer that stabilises the RO if that is a problem, not confusing their Discus Remineraliser which is just minerals, nothing to stabilise water chemistry.
And lastly... check your temperatures, its been horribly hot and while people are generally under the impression that discus like 30+ degrees Celsius temperatures... the really don't. Unless breeding, I would never have kept mine about 27 degrees Celsius. Depending on what he was used to, you might find he is too hot/lacking in oxygen?
And whilst I am still thinking on it... is he getting harassed or bullied by other fish?