Nathan's former youth basketball coach, Chris Stewart, confirmed the tragic news on Thursday ... saying the athlete was special to the program.

"Nathan was only with us for a short time in grade 7 before he moved to Florida but whenever he was back in Mississauga the boys connected," Stewart said.

"No parent should ever have to bury a child, hug your kids today a little longer. You will be missed Nathan, RIP ❤️🏀🙏🕊️"

Details surrounding his death have not been revealed at this time.

Nathan -- a 6-foot-1 guard -- played at Montverde Academy, just like RJ. He had dreams of becoming a pilot.

"In my family, we’re very competitive," RJ told SLAM in 2018. "Everyone is always trying to beat each other. Everyone is an athlete, so that’s where I get all my competitive drive."

RJ spent one season at Duke University before going 3rd overall in the 2019 NBA Draft. He played for the Knicks until he was traded to the Raptors in December.

Barrett -- who did not play in Wednesday's game against the Detroit Pistons -- has been ruled out of Friday night's contest against the Orlando Magic ... and understandably, it is unclear when he will return to the court.

The Barrett family released a statement on Nathan's death ... saying he passed away surrounded by his loved ones.

"While our family is devastated by this great loss, we will continue to cherish the memories and time spent together. Nathan was a God-fearing young man of strong character. He was thoughtful, kind, loving, compassionate, creative, admirable and driven."

"Though his time with us was brief, he will live forever in our hearts."