Masking is a learned behavior - learned specifically because the person in question had sufficiently unpleasant experiences being unmasked that they decided they needed to try hard to mask in order to avoid said unpleasant experience.

No one masks at birth. If you want your child to stop masking, perhaps it’s helpful to investigate what caused them to learn the behavior in the first place and what could be done to make the experience of being unmasked more pleasant and less aversive for him.

Just as you say, masking takes an incredible amount of energy and is exhausting and often backfires. Why would anyone expend such exhausting amounts of energy without some extremely strong motivating factor? The alternative to not masking must be perceived as exquisitely undesirable.

I think people think masking is just for those who have Autism. I think at some level all people mask. Most people behave differently outside of work or in public places when they are alone or with close friends.

I think with Autism the process of masking is just harder, and the ability to read social queues takes extreme focus to understand the general emotional state of others around them.