Mason was wearing a replica of the No. 34 jersey Allen wore at the 2017 NHL Bridgestone Winter Classic. His mother, Robyn, brought Mason to practice, where Allen flipped him a puck, signed a goalie stick and his jersey, and took a picture with him.
The two were able to chat inside the locker room at the Blues practice facility.
Allen showed him his white mask with the three Blues logos, a Canadian flag, his dog's initials and his grandfather's initials, then gave it to Mason to hold.
The two talked about the Winter Classic, which Mason attended, and the conversation turned to Allen's teammate Vladimir Tarasenko.
Allen asked Mason if he thought Allen could stop Tarasenko's shot.
"I don't know," Mason said, drawing laughter.
"You don't think so? Come on, I thought I was your favorite player," Allen said to Mason. "I can stop him, but he's on my team, so I don't have to worry about it. Right?"