Jack was photographed looking happy and smiling as he watched rugby with friends. Moments later he appeared to black out and died of a suspected heart attack.
The youngster, who practised Taekwondo at an international level, had no previous health problems.
Jack was at his girlfriend's home watching Wales play Scotland at the weekend when he died on the sofa.
Friends said he had a cup of tea at half time and was photographed holding it as he watched the match - his girlfriend put her arms around him and an ambulance was called.
But the fit young schoolboy could not be revived and was declared dead at the scene.
Martial Arts instructor Peter Kent-Marrast told the Daily Mail: "He was in front of the TV watching the game with his girlfriend.
"At half time he went and made a cup of tea and sat down and that was it, he just died.
There was no gasping or anything, he just died watching the game with his cup of tea.
Jack, of Oakdale, near Caerphilly, South Wales, is said to have been the "perfect teenager" who always had time for everyone.
Peter said: "Jack was superfit and great at all sports. He was a second degree black belt at Taekwondo, he was great at football, rugby and cricket.
His heartbroken parents Grant, 42, and June, 44, and his little brother Owain were "devastated" yesterday at the family's semi-detached home.
On Facebook they wrote: "Grant, June and Owain would like to thank you for all your support.
"We lost our beautiful son and brother on Sunday 12.2.2012. Words cannot describe how we feel our hearts have been broken.
"Jack Will Never Walk Alone he will march with Angels."
A post mortem is to be carried out.
What a sad story.