Stay duration to maintain for next family permit and UK passport for child born outside the UK

[smah]

Guys, we hope you are all well in this great difficult time.
As you are aware from our last post my husband has received his visa and we are currently in the UK.
We find ourself In a dilemma below.
Our second child was born in Bangladesh during my stay there. I am British by birth so is my parents and their parents and as far back you can go. My husband is a Bangladesh national and currently holds the 2.5 year biometric residency permit in a 10 year settlement route.
Q1. Are their any restrictions or timeframe by when we need to apply for our sons British passport from Bangladesh!!
Q2. What is the minimum time duration we have to stay in the UK for his next visa application to be successful?? And if the full duration needs to be in one stretch or it can be spread over the year?? Do me and my husband both have to be present in the UK for that period together/ individually??

All responses will be appreciated.
Also any questions to help understand the situation are welcome.