Leaving Bali with a child after 10 years (Long Post)

Hi all. Hope you are all well and in good health, I have joined this community to ask a question in the hope that someone here may be able to give some advice.

I will endeavor to provide a bit of background on a situation and then go on to ask my question. Any help is greatly appreciated, but I guess this post is aimed at the more experienced expats living in Bali.

So, on with our story...

Let's say my "Brother" was married to a Balinese woman from sometime shortly after 2005 up to around 2016 in which time the first few good years of their marriage was spent in the UK, giving birth to a Son born in the UK. They were married first in the UK and then in Bali in a traditional fashion, registered in both countries.
About a decade ago they all moved to Bali and gradually things turned sour in their marriage for many reasons but most from what I understand was the expectation and pressures created by the extended family upon my Brother's Wife as to what they should be doing from the Balinese religious perspective. This in turn created separation over a long period of time and finally divorce a few years ago.
My Brother refused to give up the custody of his Son for many reasons, including the toxicity present in the alternative Balinese family environment and so decided to leave and retain guardianship of his Son as a single Father upon separating from his wife and her family a few years before the actual divorce.
So to be clear first there was 4 years of separation and then divorce all the time of which the Son stayed with his Father, living and working in Bali.

On to the present time where the divorce has been concluded and the Father retains his guardianship of the Son by court decision in a Balinese court, he now wants to return to the UK primarily to get his Son into a good school to complete his GCSE's and enroll him into a good college with a solid apprenticeship program.

As a matter of note the Son does not like Bali. Misses England and wants nothing more than to go home. He also wrote for the court during the divorce process he did not want to live with his Mother or Balinese family and dreads going to see them each week even if just for a day.

The problem arises due to the Son entering Indonesia on his Mother's Indonesian passport 10 years ago. Although now having a British Passport of his own, the Son has NO stamp in his passport due to arriving on his Mother's Indonesian one.

Now considering the relationship between the ex husband and wife is far less than amicable, so much so they barely speak and certainly don't help one another it can be taken as such that the ex Wife will not do anything to help facilitate them leaving even if it is for the benefit of her Son's potential future.

This leads me onto my question:
How does a man leave Bali with his Son of mixed Indo/UK race if the Son has no ENTRY stamp in their UK passport?
But is a UK National and Citizen having been born and having lived in the UK for the first 4 years of his life.

My Brother gave me a couple of points he had already received from friends that I will add here:

1. That due to the Son being of mixed race he is required to have an Indonesian passport before he can leave and in order to get this passport his is required to also submit his UK passport to the immigration or he cannot even apply for his Indonesian one. And even if he were to do so he would need details like KK/KTP etc. from the Mother of which she wont provide.

2. That he can leave on his UK passport and only that, but that due to having no stamp he will be deported in the process.

3. He can try to apply for an emergency travel document from HMRC although in doing so a whole case file would have to be created and all Indonesian court documents and decisions professionally translated, which as some of you may know can really add up.

My Brother is honestly a good man, quiet and reserved but solid and determined in nature and all he wants to do is what any Father would; which is the ability of giving his Son every opportunity in life to better himself and become a good man in a world in which we currently find ourselves, surely, that can only be a good thing.

So that's the question. If anyone has any advice it will be passed on.