I was a frequent UM from age 9 upwards.
From Auckland to London your best bet would be Air NZ on a through flight. He'll have to get off the plane when it stops but there is no wandering around from gate to gate.
Although anything that had few stops and no interline changes would also work well.
Oh, and don't worry about it too much - by the time he has done the trip a couple of times he'll know what to do and will be complaining about having to go UM anyway!