Charlie Simpson set up a JustGiving page with a goal of donating £500 to UNICEF via a sponsored bike ride, but the figure has topped £150,000 and is still rising.
In a meeting arranged by The Mirror, Cowell said: "I just wanted to congratulate you on what you've done, Charlie. You're the most famous boy in the world and rightly so.
"I think what Charlie's done is amazing and we can all learn from him. He's the most astonishing fundraiser in the country. There aren't many adults who have ever raised so much money."
Simpson said: "It was great to meet Simon, I watch him on the TV all the time and he looks the same.
"I'd love to come back and see Simon again. Now I hope he remembers to give the money he promised."