I assume that since AirNZ and BA don't code share, your trip is on two separate tickets.
This might be a problem. The best bet is to ring AirNZ and ask them about the luggage being checked through. If it's not, you're going to have to clear immigration (with a visa) pick up his bag, check in to his second flight, and go through security again. This can take a long time.
What's your connection time in Beijing? Risky with anything less than four hours. These kind of trips with legs on seperate tickets can be quite risky, as if the first flight is late and you miss your connection, you now have an argument about who's liable, and it'll probably be you.
Air NZ's website has this to say:
Connecting with Other Airlines
If you have a connecting flight with another airline, your baggage may
be through-checked if Air New Zealand has an interline agreement with
the other airline. Your flights must also all be on the same ticket.
If your flights are ticketed separately, baggage may only be
through-checked if your tickets are issued in conjunction with each
other. Please contact your booking agent if you are unsure whether
your baggage may be through-checked.