Kid is going on a paid trip by the coolidge scholarship foundation and visiting white house. What do the kid wear? Pants and blazers? or just pants and shirt? How hot it will be in DC at this time?
I would have your son contact the group he is traveling with and ask. Do they have some kind of special tour set up? It will be hot.
Count on it being hot and humid. There’s a reason official DC usually flees town during August.
Will they be meeting dignitaries? Or is this just a tour?
We went to the White House and had a tour of the Capital in May. All of the ladies wore casual dresses with flats or sandals and the guys wore lightweight pants and a short sleeve shirt.
It will be hot, hot, hot right now in DC.
I remember thinking that especially for the Capital tour (which was a private tour by a member of the staff of the congressperson) that I was happy I was dressed up. Washington is a more formal dressed up city than many. Although there were hordes of school groups at all of the museums and they were in shorts and T-shirts.
If this is a formal visit where they will meet any Government officials you can’t go wrong with khaki slacks, blazer and dress shirt no tie. Formal enough to meet people, casual enough that you can lose the blazer if it’s too hot. If it’s just a casual walking tour you can wear shorts and a t shirt like all the other tourists.
I went a few years back, when you needed a connection to get in. D knew someone. We had dress restrictions, business casual if I remember correctly. No purses, bags or backpacks were allowed, so one needed to have drivers license for ID in a pocket, and a cell phone in another pocket.
I went on a specially arranged tour of the West Wing last year and was instructed to wear business attire. We were able to take our purses and phones although pictures weren’t allowed.
MAGA hat? 
Pretty sure they’d like it.
A tin hat
Red dress and white hat.
A vintage Crash Test Dummies tee? About 20 years old now…
“… Who dug the hole in the outhouse?
You did, baby - you did
Who put Whitey in the White House?
You did, baby - you did, baby…”
Traditional clothing of people from hot climates (e.g. Arabs) may be weather-suitable.