Sage, great your son is coming WWOOFing here.
Earthquakes: nz is not called the shakey isles for nothing. There is a sizable fault that runs the length of the country, a little like your San Andreas fault. It could go at any time......tomorrow afternoon or in 300 years. No one has managed to accurately predict quakes yet.
That said.....
1. The Christchurch quakes were a exceptionally rare earthquake cluster. Quakes do not normally happen like this. Normally you would get a quake, lesser aftershocks for a few days/weeks and that is it.
2. Quakes don't normally kill you, buildings do. If your son is WWOOFing he will be on a farm. Most 'none city' NZ buildings are wooden with a tin roof. They are unlikely to do you any harm. Although we have had 13,000 quakes only 1 of these killed anyone, although the others did sometimes cause injuries.
3. Even in NZ not everywhere is affected, far north and Deep South rarely have quakes. A big quake might be widely felt, our biggest was felt the length and breadth of the country, but for the vast majority of the country it feels no worse than a large truck rumbling down your street.
4. Even if a big quake hits whilst your son is in town, NZ is working hard and fast to strengthen its buildings. He will almost certainly be fine. Of course Christchurch is the safest place in NZ right now....all our unsafe buildings have fallen down.......
Tell him Bon voyage from me. I am sure he will have a blast. They only problem I can see is that he will fall in love with NZ and not want to come home......