He did not grow up on clay, that's a misconception. He spent 2 years in Barcelona. He grew up on hard courts.
With his movement being one of his key strengths, he doesn't do well on clay, because he is unable to slide into his shots. His forehand is also not potent enough to hit through the court on a regular basis. Obviously, confidence goes a long way as well.
All of this.
But particularly the first part.