He has been surprised by the reaction to the show, saying Jack was being stopped in the street for selfies and had seen his social media following skyrocket.
It had also made an impression on the personal trainer’s grandparents who were gripped by the action, he said.
“He’s a great lad, very affable, great with people,” Butler continued, adding his nephew had flown under the radar.
“He’s just enjoying the ride and let’s hope it takes him to victory tonight.
“I really hope he can see it through, get some money and treat his uncle, hopefully.”
The final episode airs on BBC One and BBC iPlayer at 20:30 GMT.