Legally, it should be the choice of the property owner, and that should be the end of it. If he wants you wearing a mask, then wear one. If he doesn't want you to wear one, then don't. If you disagree with him either way, your remedy is to leave. His property, his choice. Unless you're being held on the property against your will or unless it's a government building in a jurisdiction in which you pay taxes, you have no right to do anything. Having said that, he's a total ******* whether the kid is immunocompromised or not. If someone affirmatively wants to wear a mask, nobody should care. It's not hurting anybody.