Alex Leapai Jr has turned his back on an NRL career to finish a job his father started.

The 18-year-old heavyweight's campaign will begin in Adelaide on Wednesday when the 122kg teen faces New Zealand's Joe Ageli (2-1) in his professional debut.

Schoolboy league sensation Leapai Jr held contracts first at the Sydney Roosters then Gold Coast, before deciding to adopt his father's pursuit full-time.

Leapai Sr was the first Australian in more than 100 years to fight for the heavyweight world title, and was comfortably beaten by Ukraine monster Wladimir Klitschko 10 years ago in Germany.

There is genuine belief in Australian boxing circles that his son can go one better, with trainer Noel Thornberry adamant he has a future world champion on his hands.

"Dad achieved a lot of stuff in his career but one thing he didn't was conquering the top of the world," Leapai said on Monday.

"That's the job and my journey now is to finish that job.

"Boxing is the world stage and ... you now what they say, dream big or go home and I've got a big head."

Leapai Sr has remained heavily involved in his son's boxing upbringing but won't call the shots on Wednesday, other than to offer one piece of advice.

"Just the big thing is to listen," Leapai Jr said.

"It's a big part of boxing; if i don't listen I'll be fighting my own fight. Your coach see things you don't and Dad's experienced all of that."