Are you looking for ideas for the students or for your son to wear for the week?
I would be worried about cultural appropriation if he wears clothes that do not represent his personal family background. So the question is, what is his family background? Look for clothes that represent that.
If English, he could have fun with a red jacket, black slacks and fuzzy hat to represent a palace guard. If Scottish a Kilt would work if he has one, if not what about a tartan vest? Irish an Aran sweater. German, from the south Lederhosen, Beret and white/black striped long sleeves t-shirt for France.
I think I would take an easy way out and either applique an inexpensive t-shirt, or make a vest for each day. Or find a hat for each day.
It sounds like a great activity for the school. My university has 'World' Days in the Fall. Students from different countries had a fashion show put to music from their home country. It was quite incredible to see all the different clothes. I was especially amazed by the Korean dresses.