Discus being slugish and not eating well ??

[GreatGenes]

I have a Green turquoise he is the smallest member of my tank at around 2.5" he is not being picked on by any other discus in the tank but he isint eating near enough and looking very thin, his movements are slow and hes hanging out near the surface of the tank 95% of the time even when it comes to feeding he doesent seem fully intrested. the tank is very healthy and been established for about 3 years, all conditions of the tank are normal and the other discus look great as usual, this started about 10 days ago. what could be the issue with my lil dude ? im really worried im gunna lose him :(


Thanks
Lindsay