Hi and welcome to the forum
Firstly, sorry to hear your son has been ill, but glad to hear he was successfully treated and is now well - I can sympathise with you as one of my sons had leukaemia, but is fortunately now a healthy 17 year old
Anyway, with regards to your situation .... will you or your son be in a postion to pay for any medical costs yourself? In other words, how do you anticipate his ongoing medical costs will be paid? I know that INZ are quite strict about people coming to NZ and becoming a burden on the health system, but it does seem a pity for you when what your son requires is actually maintenance, rather than 'treatment'
Have you got a case officer you can approach for advice?