He'll have a whale of a time, Jags. I'm not a kiwi but did a 3 month trip there using tourist buses and sharing a guy's car in 2011 and loved it. I would say just go and see what happens. Once he gets talking to the locals then who knows what opportunities will arise - I got chatting to the owners of a hostel up in Pahia and spent the next 3 days helping to crew their mate's yacht down the coast to Auckland!
As I went in January there were really cheap deals on the tourist bus routes so I bought a cheap bus ticket before I left the UK covering both islands. Once I got there I used it to go around the North Island but on the South Island I started travelling with a Swiss guy I met in a hostel who'd bought a car. This was much better as we could get off the beaten track and drive up to some of the trail heads for mountain walks. If your son is into mountain hiking then there's a fantastic network of mountain huts that you can make use of.
It would have been fun to cycle as the distances aren't that huge and stopping to chat to the locals would have opened up some great opportunities. But then again, buying an old car is pretty common amongst travellers as you can pick up a reasonable one for around 1000 UK pounds, pay the road tax (which has 3rd party insurance included quite cheaply) and drive it. Then, when you come to sell it I think you need to MOT it but that makes selling it easier. Buy an estate like my Swiss friend and you can sleep in it!
One place that I can really recommend is a small hostel at the north end of the west coast on the South Island in Karamea. It's the only place I've stayed at that has its own organic farm, community radio station and artist in residence! Great place for an artist to visit.
http://rongobackpackers.com/
