Hi Patricia,
I can see both sides of the coin here. I like Matthew would challenge people not abiding by the rules. I can understand that your son doesn't feel he should have to explain to complete strangers, but we are in a very odd situation where people feel threatened by people not wearing a mask. Catch 22,
Perhaps the answer is for the authorities that issued your son with a certificate, could issue people that are exempt from wearing a mask with badges that they can wear when out and about and people can clearly see that there is a reason for them not wearing a mask, without them having to explain to strangers.
Regards Jenni