Thanks all, this is really helpful!

https://nmajmh.org/ focuses on Jewish military history. Definitely off the beaten track (tourism-wise; it's actually fairly centrally located).

The WWII memorial has ranger-led tours regularly. You can also see if any Honor Flight tours are scheduled to visit so he can meet and thank WWII and other vets.

Arlington National Cemetery.

Definitely the Holocaust Museum.

-American History museum
-Navy memorial and museum *the one by Penn Quarter
-Holocaust museum
-Iwo Jima memorial

This looks like a good writeup: https://www.tracesofwar.com/themes/3708/WW2-sites-of-interest-in-Washington-DC-en-Arlington.htm

Udvar-Hazy Center (Air and Space Museum) near Dulles. The Air and Space Museum on the Mall as well.

National Museum of the US Navy - at Washington Navy Yard in DC

National Museum of the Marine Corps - in Triangle, VA, south of DC

Holocaust Museum

Air Spac Herndon. Spy Museum.
Torpedo Factory Alexandria

My brother-in-law is coming to DC for the first time in July. He is really into WWII stuff. Other than the WWII Memorial, are there other sites/museums/tours you'd recommend?