Poor guy. I wonder if he couldn't fit the pellets in his mouth or something and that's why he didn't eat the pet store.
You can get some fresh garlic and crush it up, then mix the crushed garlic with a bit of tank water. Or you can try dissolving garlic powder (no salt) in tank water and see if that works. If he WILL eat the garlic-soaked food, the added benefit is garlic will boost his immune system and help fight off any illnesses he may have.
He's a really pretty iridescent color, I can't wait to see him when he's healthy and colored up.