He played in a junior world cup for his country and against a New Zealand schoolboys side that included the likes of Joe Rokocoko and Liam Messam.
Thomson's career was cut short at the age of 22 when he tore the ACL in his knee three times.
That has not diminished his love of rugby, however. It runs in the family after all.
His dad, a huge fan of the oval-ball game, has guaranteed lifetime seats at the home of Welsh rugby in Cardiff.
"So I've been lucky enough to go to test matches in Cardiff Arms Park, or National Stadium, or Millennium Stadium since a very young age," Thomson said.
His old man was at the England and Wales game at Twickenham on the weekend and phoned him straight after, while still at the stadium.
"They were very happy, all I could hear was a lot of noise. He was crying I think ... at the final whistle."
He watched the Wales and England game at home with friends, including with another Welshman and an Englishman.
"It was pretty intense ... the English boy was reasonably confident - which you would be 10 minutes to go - and he's pretty shell shocked [after the game] and me and the Welsh lad, we didn't know what to do."
As for watching rugby in the Welsh capital, Thomson said it was a "pretty special" experience.
"When Wales win, there's nowhere else better really."
Cup super fans
•Today: Wales.
•Will play: (Pool A) Uruguay (won 54-9), England (won 28-25), Fiji, Australia.
•Ranked: Second.
•Team logo/emblem: Prince of Wales' feathers
•Anthem: Hen Wlad Fy Nhadau (Land of My Fathers)