My son. now 48, also is dual Thai/British. He does not read or speak Thai, has never lived in Thailand since he was 13 months old. Married with family lives and works in the UK. His Thai current PP has expired and, I think, his Thai ID card. and he has no plans to renew them. He is not registered in any Thai Tabien Bahn (House registration book)
With no reliable way to track his location he has never been called for Thai National Service and is now too old anyway.
With your son being registered on a Tabien Bahn his details must be on a database with that house shown as his residence and I should expect any summons to be delivered there.