Important reminder: different immigration offices have different policies. This post is about Bangkok Immigration. Information about Kanchanaburi (for example) isn’t helpful.
But he entered on a 90 day visa exempt, and then got a 60 day extension of stay based on unable to travel due to COVID-19. Some immigration require it, others don't.
1. NOBODY knows if the 60 day extension of stay based on unable to travel due to COVID-19 will be extended or not. It has not been announced yet. We *WILL* Make an announcement if it's been extended.
2. Different immigration offices have different policies about acquiring an extension of stay. Chiang Mai, Bangkok, Buriram, Chiang Rai and a couple of others all allow people to apply for extensions when they have 45 days or less left on their current stamp. The other immigration offices are 30 days. However, ask your local immigration office for specific details.
3. If you have further questions about the 60 day extension of stay based on unable to travel due to COVID-19, please read
You will contact your embassy, and ask them for the document. Different embassies have different documents, and are called different things based on local language.