I went to a boarding school in Ireland for a year, and they had a uniform -- but it was a uniform, not a "policy". There was one pair of trousers, one shirt and one pullover allowed, and they were provided by the school.

This was fine with me; it was a very clear rule with only one interpretation possible, and it was completely external to my relation with my parents or in fact authority in general. You had to wear a uniform, you wore a uniform. I was quite rebellious but I never had a problem with that.

My guess is the OP's kid resents arbitrary rules and the way they are enforced. At what point does "dark blue" cease to be dark? And why does this woman feel the need to enforce the school's rules? Why doesn't she let him see for himself what happens when you go to that school with a red pair of pants? Does she fear it will reflect poorly on her?

I feel everything is about her, not him, and that's why he's so upset.