you are in luck, London has no seasoning requirement. They want three months bank statements to show income but require £11,000 (500k THB) only at the end, not for the duration. I got my DTV there and only had the full amount at the end of the period I sent them.
in this case his son is in the house, that's where the human right aspect comes in. Many countries do recognize the right of child to live with their parents as a human right. Thailand has a visa for it as well, but there's a court process to certify his parentage as he's not married to the mother.
yes, it's convenient if you don't have your card with you as well. You go to cardless withdrawal in the app, then say how much you want, and then select it on the ATM, it gives you a QR code you scan with your phone and then it dispenses the money. It works the same in your home province as well, it just has this advantage that it works throughout Thailand with no out-of-province surcharge.
he needs to get a court to certify that he's the father of his Thai kid. Then he'll be able to get a long-term visa on that basis, until the kid is 20.
He has a Thai child. In many countries including Europe family reunification is seen as a fundamental human right.
Thailand has a visa for this too, but my understanding he needs to get a court to certify the parentage if not married to the mother, this may be the stumbling block. There are also the financial requirements (as many countries).
the issue could be that they don't come in as international and so wouldn't be accepted by immigration. AFAIK GSB isn't a direct Wise partner so they will *never* come in as international there. You can get documentation from the bank they come through in this case but that might have been what they were trying to explain.