He needs to get back onto a ‘Retirement Extension’. Given his financial situation, that means paying an agent each year. Lots of people do exactly the same.
Cost will depend on if he has a Thai bank account and which bank it is with. Bangkok Bank is no longer an option for agents.
Assuming he is allowed in at the airport on Visa Exempt he can then go to an agent and they will sort things out.
If he doesn’t have a suitable bank account then the initial cost will be
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baht depending on agent.
Subsequent years will be much cheaper. Maybe
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baht each time.
Many, many people living here do this.