I think it's reasonable for your son to "remind" them. They are his tenants and if they were littering outside the building for example, or leaving the bin in the wrong place, their actions would impact on the local community. I'm not saying he's responsible.
Over here, when I worked in social housing, we had a responsibility to all tenants to be living somewhere safe - and also obligations to inform them about local rules and regulations. You could argue that other tenants are at risk if one is breathing all over communal hallways, handles and so on.
I haven't kept up with housing law but there was also a clause in the standard contract about not letting your premises be used for illegal purposes (think cannabis growing, or as a brothel) because this technically had implications for the landlord, not just the neighbours or the tenants themselves.
Maybe someone here knows more about this than I do.
Anyway, if they are behaving technically illegally by being outside without a mask, they may be in breach of their housing contract so your son may be a good person to remind them of their obligations to the community.
I can't believe he would be responsible for them or nobody would ever let a property to anyone!