M.H.
When my daughter was in 1st grade they did a play about the American Revolution. Most of the boys wore dark baseball pants and white button down shirts of their moms (not that any of them would admit it!). Don't know if that would work for your son or not. Many of the girls just wore long skirts (or again, raided mom's closet and safety-pinned at the waist), a button down blouse, and threw on an apron tied around the waist. I know 10 year olds are pickier than 7 year olds, but hope this helps!