I think this child is at risk using the boys bathroom and should be using the accessible washrooms or the teachers washrooms.  I think a child's safety is more important than their preferences. I don't think this student would be safe without an escort to the boys washroom, and I have taught high school.  It's a simple solution and a safe one. And sometimes children's desires need to have an adult intervene to let them know it isn't safe or practical. They don't need to be forced to use the bathroom of their birth sex, but probably should not be entertained at will either. Adults can take their safety into their own hands, not children.