Thanks for your question.
If there was a break in cohabitation then the this will break the time required for the purposes of an unmarried partner's visa.
There is no alternative visa which he can apply for under the standard immigration categories. Discretionary applications outside the immigration rules are permitted though my advice would be to instruct a local solicitor to prepare one based on his Article 8 Right to a Family and Private Life.
This would argue that he should not be compelled to return to NZ on the basis that his, your child's and your right to a family and private life would be prejudiced if he were. You would argue that he is required here as an intergral part of your family.
Coupled with the fact that he would in 6 months be eligible for an unmarried partner's visa this may get some traction witth the UKBA to the extent that you would have a reasonable chance of success if properly argued and evidenced.
You should submit this application as close to the date of his current visa expiry as possible. The UKBA aims to reach a decision on such applications within 6-8 weeks, but frequently overun in the case of discretionary applications. He will have leave to remain until determination is reached.
if it is rejected then you can appeal., It takes around 2-3 months for a hearing date to be set so by that time you will be nearing the requisite cohabitation period for eligibillty of unmarried partner's visa so I would hope that the UKBA take a pragmatic view and either allow your application before appeal or, if they don't, you can submit an application for an unmarried partner's visa before appeal that should be sucessful.
Even if you are rejected at appeal he will, I should think, only be a month short of the further 6 months he requires. He could overstay to reach the required time and then return to NZ breifly with his unmarried partner's visa application in hand ready to submit straightaway. Under rule 320 of the Immigration Rules the UKBA are not permitted to refuse an application on the basis of an overstay alone, so provided there are no other defects in his application and it is properly evidence he should obtain an unmarried partner's visa.
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EDIT:- ps. I have to go offline for an hour or so now, if you require expansion or an answer to follow up questions I should be able to respond upon return.