New Zealand Law
Have New Zealand Law Questions? Ask a Lawyer Online.
In this situation I believe the Immigration department will accept the most recent certificate. I don't know if it is possible for you, but if he has a balance sheet for his own business which shows the income, that would likely also help as well. So if he applies in about the same time as you or a few months after, that certificate should be reasonably current.
The rules are not so precise, but it just has to be credible evidence of his income which is as recent as possible. It is certainly possible to come as a visitor, and then make the parent application. Sometimes that is easier. As I typing this I see your follow-up, and it certainly would make sense to have you apply for him soon after you arrive, because these do take a bit of time to process.