Dr O'Sullivan said he and his wife called a whanau meeting to explain the condition to their other six children once they heard the diagnosis from a specialist, Radio NZ reported.
He said Lance Junior was not able to walk as far as other children when he was a youngster, falling over a lot and struggling to keep up on the rugby field.
Duchenne muscular dystrophy is a genetic condition that involves muscle weakness and worsens quickly.
It affects 1 in 3500 births in the world, largely boys, and leads to progressively worsening disability.
Death usually occurs by 25 typically from lung disorders, the US Library of Medicine states.
