However, it turns out he’s been charming viewers for a lot longer than most of us might realise.
Following the success of It’s A Sin, fans have dug out Callum’s audition for the BBC reality show Let It Shine, which saw hopefuls trying out for a spot in a new musical based on the music of Take That.
“I go to a school called Treorchy Comprehensive School,” he tells the panel. “And I’m Head Boy!”
“OK, [I’m] American,” former Glee star Amber responds. “What does that mean?”
Callum then explains: “So it’s like, there’s a country next to England, and that’s where I’m from.”
As the audience erupts into laughter, Amber then clarifies: “No, no, what does Head Boy mean?”
Following his intro, Callum then goes into a rendition of You’ll Be Back from the musical Hamilton, winning over the judges and earning himself 16 points out of a possible 20.
However, Callum clearly had a change of heart after his success in the audition stages, as he ended up bowing out of the contest before it went to the live shows.
That might have been a wise decision, all things considered. It’s A Sin has introduced Callum to a whole new legion of fans, with the show giving Channel 4’s streaming service All 4 a record-breaking month.
Since the whole series began streaming on 22 January, 6.5 million have tuned in, bagging the platform its highest monthly figure since 4OD launched in 2006.
It’s A Sin is available to watch on All4 now, with episodes also airing weekly on Channel 4 every Friday night at 9pm.