Can my son apply for a visa exemption extension in Hua Hin if he registers our Bangkok address on the TM7 form?

To apply for a visa exemption extension, your son must do so at the immigration office corresponding to the address on the TM7 form. Therefore, if he registers your Bangkok address, the application must be made at the immigration office for that area, specifically the IT Square Laksi Plaza office. If you are in Hua Hin and prefer to apply from there, you can use the hotel address and ensure to obtain a TM30 receipt from the hotel.