A n-95 is better but a kn95 is better than nothing. Closing the vent or turning off the heat when he’s working is a good idea. Personally I would be more concerned with the regular dust particles than with any asbestos. It sounds like thats what gave your son a stuffy nose.

Even if there is a positive result on the asbestos test I still wouldn’t be very worried. There are guys who work with old drywall every day for decades and never develop any problems.