The 23-year-old had already landed a few big blows before finding the mark with two big rights, which forced the referee to immediately step in as the Italian crashed to the canvas.
"There is only one pro debut and we ended it in spectacular fashion," he later told Sky Sports News. "You could see he was game, he could take a shot.
"It's just the start though, I won't get carried away. It's just the beginning."
He added: "It was just as important as winning the Olympics as everyone is here to support and everyone is proud that we got the victory and it's just onto the next one now.
Leo was previously unbeaten in eight fights.