the general medical insurance seems sensible to me. many old foreigners get seriously sick/die in thailand without adequate insurance and leave thai taxpayers to foot the bill. but as long as quarantine remains in place i can't see the need for covid specific insurance.
just go to any big mall with your passport and whatever other documents you have (e.g. work permit, residency certificate, property ownership/tenancy papers) and go around each bank until you find one that says yes. if none of them can be bothered with the hassle (it's possible but unlikely) drive around the block to the next mall and repeat.
he will undergo 2 weeks of quarantine on arrival in thailand (effectively isolating himself from the human population) and be tested multiple times. if you go anywhere in public OR don't have a test in the next 2 weeks you will have more (probability adjusted) contribution to the spreading of the virus than him.
No vaccine is 100% effective and Thailand isn't willing to accept any risks. They won't allow anyone in without quarantine until the country they are coming from has been covid free for a while.