Pictures of your boy would be of great help.
How is his swimming? Is he fairly active? What are you feeding? Sometimes fish do not realize what we offer is food if it is something other than what they've been fed. It takes a while for them to figure it out. I'm not one for giving in, but if he is already physically compromised you might 1) try frozen bloodworms and Brine shrimp; or, 2) ask what the store was feeding.
And I mean frozen; not freeze dried. To feed, scrape off a bit, let thaw and then offer using blunt tweezers or an eye dropper.
Let us know what else you need.
And....Welcome to the Forum!
If your dog thinks you are the greatest do not seek a second opinion