The Deadpool actor first met Connor and his family through the Make-A-Wish foundation three years ago and the pair became close friends before he died earlier this week.
Paying tribute to his "great friend", Mr Reynolds wrote on Facebook: "For three straight years, my friend, Connor McGrath drop-kicked cancer... Not sure how... Maybe the cancer cheated... But the fight came to an end two nights ago."
He added: "He went way too early and it's impossible to reconcile. Connor was a great friend, a great son, and a light to the people lucky enough to know him."
Read his entire post:
Mr Reynolds also revealed that Connor was the first person to see Deadpool after he surprised him with a rough cut of the movie before its release and the actor believes that his late friend embodied all of the best parts of the comic book character.