Call ahead to see if they can Waiting to 12th you may not even be boarding. Buy a new flight, make a new plan, or take your chances. If you actually made it to BKK I think your chance is good. Much easier to call somebody to print your boarding pass than sending you back!
That is even better. These third party websites are selling “illegal” connections in many markets. Even JAL and ANA genuine sites have been selling Haneda+Narita connections and international connections to domestic flight. Basically not allowed due to covid restrictions
What it does not say. You need a connecting ticket on a single itinerary. Getting Thailand pass for a two hour connection is not an option. ⬆️ . If the airline in Columbo even lets you board there is a chance you can get the staff of Lanmei airlines to collect your bags and passport from airside then issue you the boarding pass. Otherwise you would have to be sent back to Sri Lanka. About six months ago a woman from Burma was able to do this to get to UK.
There is no way you can collect your bags yourself unless you meet all the requirements to enter Thailand. Then you must go directly from arrivals to the Hospital (or your test and go hotel if they have onsite testing) , you will not be allowed to go to the 4th floor to check in for another flight.
Some airlines do not have interline transfer agreements. If they even let him board it is possible, to have the connecting airline staff collect his bags and passport and issue a boarding pass from airside.