This rule will take months to come into effect possibly it won't even happen who knows if you are traveling soon you will get stamped in for the original 60 days then go extend for a further 30 days at immigration for 1900 baht
If he was Thai Born why would he have a problem entering his own country my daughter is Thai Born and holds duel citizenship to Thailand and my country which I won't mention
No can only extend once if you arrive on a visa exempt for the 1 time visa extension you need TM30 form from your hotel or landlord where you are renting if you want to stay long term contact your local embassy in your county for different visa's
It should be on record that you checked in or paid rent depending on where you're staying ask them to download the confirmation form they should do it online anyway