He can self file at Phuket immigration. Ask for the handout and don't forget the stamps for the power of attorney. There is a small food kiosk at immigrations which sells the stamp.
Also when you've got a new visa. I haven't changed the address but came back from overseas with new O-A visa. Phuket immigration insisted on new TM30 before 90 day reporting.
For issuing a O-A you can use a foreign insurance during the 1st year and you should be stamped in accordingly . In that case it should have been 89 days ... 🧐