I went to a school that had uniforms, and VERY strict rules about how the uniforms were to be worn. Tie knotted in a Windsor or half-Windsor, top blouse button done up, knee socks pulled up to the knees, specifications about the colour and types of sweaters, socks, ties, hair accessories, earrings, hair styles, makeup and nail polish we could or could not wear.
I didn't like all of the rules, but I could have changed schools to one with no uniform, or a less rigid dress code. I chose to stay, so I chose to be bound by those rules.
Those parents chose to stay, but they're choosing not to have their son be bound by the rules.
Are the rules stupid? Perhaps. But since they exist, they need to be followed. If necessary, the parents can advocate for the rules to be changed, or for an exception to be made for hair grown in support of that cause, or some other way. However, I don't think that having their child just break the rules is the right approach. What does it teach that little boy about dealing with the many rules and authority figures he's likely to run into throughout his life?