Once he gets his Thai ID, he should be able to remain indefinitely, even if he enters on a British passports, isn’t that right? What I don’t know is whether he would need to do some sort of border run to reenter on his Thai passport. For context, his Thai passport is expired, but he will renew it in Thailand when he gets his Thai ID.
thanks Graham! I’ve done lots of research, but finding it difficult to pin down whether my thoughts are correct, hence always worth asking in these groups in case anyone else has experienced similar!