Boxon captain Kalani Matthews led from the front with two quick tries to get them back in the game.
After Boxon scored again they needed Kalib Houltham to nail a pressure conversion goal, which he did to send the game into extra time.
Playing an extra five minutes each way, the decisive try came from Tumanako Kumeroa Woon, who took the kickoff for the start of the extra time second half and broke through several tackles on a 65m dash to the line.
The victorious Whanganui Boxon's first order of business was to go find their team mate Liam Simon, who had been taken to hospital after popping his shoulder out.
Campbell said the club was delighted for the boys' success in the six-team competition, pointing out the effort it took for them to travel by van to Palmerston North every Friday night for games so a number of them could still play Saturday rugby union.
It was a schedule that took its toll on the Boxon's Under 17 team, who did not complete their season.
Campbell also praised coach Dominic Patea, who was in charge of a league team for the first time after a background in coaching touch and basketball.
"At that age of 15 it's hard to keep them concentrating and he does well," said Campbell.
About half the team will play in the under-15, 70kg weight grade again next year, while the rest move up to Under 17s.
Matthews, Houltham, Corey-Lee Robertson, Wiremu Hiri and Tawhiri Marakau have all been named in the Manawatu Mustangs Under 15 representative team.
