S1 / Z-visa

I came to China for Work (Post Doctorate) in April 2018. In my work-Z invitation letter, they also mentioned the names of my wife and my daughter, but now we have another kid (son) but he is not added in that list.
I am now already in China but the work-permit department of Sichuan (China) is not adding our newly born baby in invitation letter because they said that they issue letter once only.
My family is in Pakistan and they want to come to China.
What is the solution for our new baby ?