This is the partner-sponsored residence visa, in detail. See the button at the bottom of the page for the criteria and evidence you would need.
https://www.immigration.govt.nz/new-...-resident-visa
Once he is a Resident (and he may well be granted permanent residence straight away because of the length of time you've been together), he would be able to pay domestic rate fees for his studies, but he wouldn't be eligible for any NZ government student loan until he has been a resident for three years.
https://www.studylink.govt.nz/produc...ndex.html#null