I’m glad your son was not traumatized by the experience. I have always thought it ridiculous that hair length is in any way associated with gender. My cis male partner has long hair, and I’ve dated numerous other cis men with long hair (though I do live in San Francisco).
Your story is a good reminder that while the bathroom battles primarily target trans women and girls, transmasculine people and “gender non-conforming” men and boys are affected as well, and not just in the South. I am a trans male with very short hair, but I still get misgendered as female an uncomfortable percentage of the time. And unfortunately, I cannot use a urinal* due to my current anatomy. So I am very nervous whenever I use a men’s room, where I have no choice but to use a stall. If someone were to spy on me in that situation, I would be in a very vulnerable position.
*Yes, there are various prosthetics and devices that I could use, but that detracts from my point here :-)