Nothing special to do. Your are now in Thailand. Create a new account and add this account to Google play and then you're able to switch between your accounts.
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.