You can look at Flightradar24 to get an idea of what was still flying today. 60% of departures have been cancelled but there are still more than 2/day to Bangkok. There used be something like 50-60, though.
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I have seen suggestions there may be a mandatory 14 day quarantine if he moves province, someone reported that coming back to Chiang Mai from Bangkok. This may not be self-quarantine, they wanted to quarantine him in government facilities (at his cost). But maybe OP wants to get there to fly out home or something.