Hi, I am in the similar situation.
My wife gave birth two months ago, in that time my application was in IPB office, and still is.
You will need to send following
The original or certified copy of his Birth Certificate and Passport,
A completed INZ1001 form (Additional Dependants for Residence in New Zealand)
A completed general medical form, INZ 1007 meds,
Children
• All children including babies must have an immigration medical examination.
• Children under 11 years of age do not need a chest X-ray unless the physician declares it is necessary or one is requested by Immigration New Zealand.
• Children under 15 years of age do not need a blood test unless the physician declares it is necessary or one is requested by Immigration New Zealand.
Of course, you should inform your CO if you have one, otherwise just inform any person there in Shanghai Branch and he/she will add this information to your application to notify future case officer once it has been allocated.