If diet, water chemistry and temp are fine then it's probably just genetics. He's probably from a line that had more gold than normal and it's just now showing up.

I have a pond full of fancy goldfish, including three black moors. Goldfish start out black or brown and change colors later. This helps them hide while they're small enough to be eaten. I've had babies change colors while they are an inch or so long, and others that have taken three years to change.

Black moors usually keep their black color, but there are other colors of telescope-eyed fish and I suspect that's what you have. It probably just took him a while to change.

it is normal for Black Moors to change to gold as they age. They do it at different rates but almost 100% do change.

My Black Moor is almost Gold maybe a 1/2 inch is black up top