See my #6. The school would have been opening up a huge can of worms, if they had allowed him to wear something other than the cap and gown.
This is a town of about five people total!!!!!
No, the school could make a rule that the wear for Graduation is Cap & Gown or for those who may have enlisted in the Armed Forces of the United States, the dress Uniform for their branch of service. Covers Army, Navy, Airforce, Marines, Coast Guard and doesn't allow for "exceptions". Such a rule could have been made in advance and would eliminate the "uniforms" of the Rainbow brigade or whomever.
Rob