To clarify here , you are entering under the AQ scheme , NOT the sandbox or test and go scheme , correct ? If that’s the case from here it only requires a negative covid test for test and go and sand box ,
If you are vaccinated and boosted and regularly tested there’s near zero risk you pose to yourself and others . This is likely an endemic disease unless wealthy nations donate more vaccines to poor nations. Your comment might make more sense if Thailand had low vaccination rates , but it certainly doesn’t.
You should be fine but try to see if there's an official document or notice from the UK that explains the two passports. I have found having extra documents is always better.
You or your son will likely need a certificate of recovery from covid-19 because you can continue to test positive for up to 90 days after infection. You will not be allowed to board a plane to Thailand or Singapore with a positive covid19 test unless you have that certificate. The Thai pass is only good for 72 hours of changes so you will need a new one.