Living together does not have to be in your own place; you can be living with family, friends or flatting. But living together is a key part immigration look for as proof of being a committed couple in a stable long term partnership/relationship.
If you are living together with his family, that can be used as the start of your partnership for Immigration. So if you will live together for less than 3 months in Japan, he could come with you on a visitor visa and apply for the partnership work visa once you have lived together for around 3 months. As JandM mentioned Immigration do not have a set minimum period but 2-3 months together seems to be the minimum others have successfully had prior to application. The longer you have been living together the less risk Immigration could see with your relationships stability/strength.
He may need either a return ticket or possibly proof of funds for a return ticket to enter NZ.