British Parent not giving Consent for MN1 Application

Hi guys, (new to the site, please be gentle

Here is a summary of my sons situation,

British mother (by decent) she was born out of EU but her father is British.

I am currently on parent visa 10 year route 4/10 years completed.

Our son (7 years old and born out of the EU) he currently has ILR for 6 years (yes I've been battling for a very long time with this)

His BRP and south African passport currently has expired and he has not be out of the country since granted ILR.

I am currently in the process of gaining full PR as his mother is a drug abuser unfortunately. My son lives with me full time and almost his whole life has been in the UK.

However I have pretty much completed the MN1 application for my him, but his mother (British parent) is refusing to sign consent (no reason given), wont give 3 years proof of residence and wont give forward her passport. So pretty much she wont give any information or documentation for his registration application.

Any advice would be very much appreciated. I understand ILR does not expire (unless he leaves the UK for more the 270 days or other special circumstances), so should I be patient and wait until I have finished my immigration journey (another 6/7 years until my citizenship ). He will then be 13/14 years old, or is there another way around this unfortunate situation? If so what section would you recommend 3(5) / 3(2) or any other routes?