"...a major WW2 military buff." In that case, I would suggest staying in Germany to focus on the several war sights in Berlin, Munich, Dresden and Hamburg or Frankfurt as the 4th city.
a..In Berlin see the Invalidenfriedhof ca 30 mins walk from Berlin Hbf, the oldest and most famous Prussian-German military cemtery, he'll recognise some of the famous WW1 and 2 personalities buried there plus monuments to others.
b... the RAF war cemetery on Heerstrasse of British air crews shot down during their strategic bombing offensive.
c....a day trip out Seelow, to see the museum and memorial to the 1945 Seelow Heights battle, the biggest ever fought on German soil, accessible by taking the train from Berlin Hbf to Frankfurt an der Oder, then the S-bahn to Seelower Höhen Gedenkstätte
d.. the museum in Berlin-Karlshorst (the terminus for on the S-3) from Berlin Hbf., ca 20-25 min walk along Treskower Strasse, the name he'll surely recognise. Or, you can catch the bus at the Berlin-Karlshorst S-Bahn station that go to almost in front of the museum. Outside of both #2 and #3 are numerous WW2 Soviet tanks, SU types, artillery pieces, maybe even a Katyuska truck (can't remember exactly)
e... Not to be missed is also the German Hist. Museum on Unter den Linden, where you'll see the 88 gun.
f.. In Berlin are 3 Soviet WW2 war cemeteries and memorials...the Tiergarten on Strasse des 17. Juni, formerly known in Prussian history as "Charlottenburger Chausee." The other two are in Berlin-Pankow and the big one in Treptow.
g...If he is interested in WW1 too in Berlin, see the Rathaus in Spandau, (the U-Bahn goes out there), the memorial plaques can be seen there. I finally went to Spandau as a day trip in 2017. You get the feeling you're not in Berlin ( just me) but I don't have that feeling when in these other former suburbs, eg., Karlshort, Köpenick or Charlottenburg that all joined in 1920.