This is why I refer to my wife as my ex-girlfriend.

Well he was a child prodigy, but he is no longer a child. A suitable replacement would need to reword the sentence to be about the same length and include that detail without the odd sounding wording.

"He was a prodigy. He still is, but he was one, too."

Very challenging to word in a succinct manner.

How do you feel about "former child actor"?

Yes, child prodigy is entirely about being a child.

Every year we hear about some kid who is "smarter than Einstein and Hawking" but for the most part, we never hear about them again. Child prodigies that turn into extraordinary adults seem to be rare. If you were to ask me to name some, the only one I'd be able to name off hand would be Stephen Wolfram. That here is another one is of interest even if it is of little consequence to his current accomplishments.

It's a little weird to call a 43-year-old a "child prodigy", yes. The phrase is left-associative.

Interesting take so upvoted, but calling him a child prodigy does not work as he's not a child.

Maybe "a child prodigy in his youth" would be both precise and succinct enough, but at the same time language is for humans and I feel humans know what is meant by former child prodigy.