I don't think you will be allowed. The second there is one case here everything will go into a big panic. They are not going to be able to track 200 planes a day of people into quarantine like they are doing now with flights organised through embassies. They are simply not going to accommodate getting their own citizens back and then change it for bus loads of tourists
you have no chance mate. People living here do not see it being possible for general tourists until after a vaccine (if there ever is one). If you (people in general) don't want to take it I can't see them being allowed in. Why doesn't he visit you as he would be able to enter back to Thailand soon I think if he has a visa to be here but will have to quarantine at a dedicated hotel. I think that will be your best option to look into
I think the hospitals usually have staff to apply for the medical extensions. My work colleague father had an emergency operation when making a yearly check and the hospital (Sukhumvit) sorted it out for them as he couldn't travel for a number of weeks.