If he wants a diagnosis he should take his fish to a vet then. Sometimes the process of elimination is all that is left to us 'lay' folk.
Everything hasn't been eliminated though, yet.
I don't want to come across as a complete **ck candyman, and if I'm wrong I sincerely apologise, but it's as though you have seen a pretty fish or a picture of a pretty fish and just bought it. You don't know what they were fed on which means you didn't see them feed, which leads me to believe they were bought online. If they weren't bought online then it's a plain and simple schoolboy error.
Discus are expensive, as you know, would you buy a 200k car without testing it or a million dollar house without viewing it? Add in to the fact this is actually a living thing and you have to put even more effort in.
It's quite clear they're not too keen on flake, so stop trying. Get down to your local petco or whatever it's called and try a few of the frozen foods they have. Beefheart, shrimp etc Ones with garlic are supposed to help wind their appetite up too. Or if you can't get there, just try your local shop and buy a bag of frozen shrimp or fresh liver, beefheart etc Just chop a bit up finely and try them. "Real" food anyway.
As Hans as said, it may be too late now but hopefully not. If you can try them on some real food ASAP you can rule this out one way or another. You wouldn't start dosing your kid up on medication if they didn't like broccoli would you 😀