Appropriate dress is the cap and gown. That is Tradition and Marines are taught to respect and practice tradition.
Wearing anything else is inappropriate.
It would be different if they had ROTC cadets graduating in uniform, but they did not.
It isn't Parris Island graduation day, it is High School graduation day. He had his Marine Graduation, that special day is over.
A good Marine does as authority requests; in this case wear the cap and gown.
I went to both of mine in the appropriate clothing, though high school came first and Drill Instructor Sgt Cooper, the meanest man who ever lived, didn't give me a choice of what to wear while at Parris Island. Ha ha ha.
