When I divorced my first wife 8 years ago she kep hold of the kids Thai passports despite the fact I had custody. She told the court they were lost in the tsunami. The fact that the kids always travelled on UK passports so why who the Thai passports have been lost was lost on her legal team.

Now my eldest son is 18 how can he get his Thai passport. It will have expired now unless the ex renewed it.

Should he just write to the Embassy in London explaining his mother retained it or simply say it was lost?

Get a police report to say it was lost. He'll need his Thai birth certificate and his ID card if he doesn't have it.

Note, age of majority in Thailand is 20, so I suspect he'll still need both parents signature