"He's the greatest!" Reynolds said.
"And I'll tell you one thing about that kid is he is hitting an absolutely tiny target to hit, every day on that set. I'm so proud of him and I just love him so much."
Reynolds made the comments while promoting his new film The Hitman's Bodyguard.
Reynolds revealed that Dennison's casting in Deadpool 2 came about partly as result of his friendship with Waititi, who played Reynolds' on-screen buddy in the 2011 Green Lantern movie.
"I'm very good friends with Taika," Reynolds said.
"So that's kind of how I found him. Obviously I saw Taika's film and I was like 'Who is this kid and what voodoo did you put on him to make him this good?'.
"And Taika said 'He just is that good.' He was the only one we ever considered for the role."
The exact nature of Dennison's role in Deadpool 2 remains under wraps, but it's not hard to picture the irreverent franchise making full use of the young actor's unrivalled ability to dish out deadpan sass.
Reynolds spoke to the Herald before tragedy struck on the set of Deadpool 2 last week with the death of stuntwoman Joi 'SJ' Harris.
Global News reported that on Monday Harris lost control of the motorcycle she was riding while filming a sequence for the movie, causing her to crash the bike into the glass-enclosed lobby of the Shaw Tower in Vancouver.
After the tragedy, Reynolds posted on social media: "Today, we tragically lost a member of our crew while filming Deadpool.
"We're heartbroken, shocked and devastated ... but recognise nothing can come close to the grief and inexplicable pain her family and loves ones must feel in this moment."My heart pours out to them - along with each and every person she touched in this world."
Deadpool 2 is scheduled for release in June 2018. The Hitman's Bodyguard is out in New Zealand on August 31.
