Normandy from Germany

To get some good suggestions it would help if we knew where in Germany you'll be coming from.

If your son is anywhere near Ramstein it's not hard to visit Normandy. But it does take some time, as it's a full days drive to get to the beaches. I'm not too far from that area myself and posted some of my trip notes from a week in Normandy on this thread.

https://community.ricksteves.com/travel-forum/france/brittainy-normandy-loire-and-paris-itinerary-guidance

That may give you some idea of the time requirements and what's out there to see and do. There's a lot of info on this site, and a little searching can assist your planning.

But, if you can give a general idea as to where he's at it will help.

Agree that it would help to know where in Germany your son is stationed.

Prioritization: Until details on location are available you can browse through the top 100 sights and attractions in Germany (voted by travelers).

Normandy is a long way from most parts of Germany. Lookup train journey planner (also from / to Germany): https://int.bahn.de/en

Whatever "traditional BNBs" are for you; in Germany we also have smaller hotels and accommodations with moderate places. The word "garni" in smaller accommodation names indicate breakfast option.

In a perfect world, we wouldn't be getting on a plane but if we have to, we will.

No Need. Germany's rail system is extensive and highly developed. You can reach Caen, FR by train but may want to get a car there for visiting the Normandy beach towns.

Also, any opinions on what to prioritize when we visit Germany would be great.

You made your priorities clear. History surrounds you wherever you are... Much of Germany was once part of the Roman Empire. It happens that Germany's oldest city, Trier, was an important Roman outpost with plenty of history you can get eyes on even today. And Trier is near the French border, less than 4 hours from Paris by train. Rick Steves covers Trier. More info on Trier:
https://www.trier-info.de/en/places-of-interest

The Mosel River runs through Trier and the whole river is dotted with old-world wine towns and castles from Trier to Koblenz. This is one of Germany's most scenic train routes. Sounds like somewhere you would like. Most towns have train stations; those that don't have bus connections to nearby stations. Biking along the river from town to town is and easy and popular way to take in the scenery.

Mosel River scene:
https://www.zughalt.de/wp-content/uploads/2011/10/DB11321.jpg

Cochem:
https://img.fotocommunity.com/reichsburg-cochem-mosel-d3c1f03b-fafb-44ea-ae46-aa69ba7ebdb7.jpg?height=1080

Beilstein:
https://img.fotocommunity.com/beilstein-an-der-mosel-271c065b-2100-4db9-98ef-90339e58e39d.jpg?height=1080

Mosel towns and villages:
https://www.mosel-inside.de/en/travel-guide/mosel-villages.html

Are traditional BNBs a thing there?

Fewer and fewer in number as the years pass. You can still find some homey places that come with breakfast, often in hiking/biking areas where outdoorsy types need simple accommodations - areas like the Mosel Valley and the Rhine Valley (which meet in Koblenz. See map.) They are called "Privatzimmer" or "Private Gästezimmer" or sometimes just the broader "Gästezimmer".

https://www.orslev-cykelrejser.dk/upload/images/cr/tyskland/mosel/model%20og%20rhinen%20-%201%20ny.png

Here's a Privatzimmer in a Rhine Valley town called Osterspai, near the Rheinsteig hiking trail:

http://www.metz-braubach.de/html/bilder.html

There are also Guest Houses ("Gästehäuser") operated privately or by businesses, like this one in a local bakery in the Mosel town of Bullay:

https://www.cafe-baeckerei-goergen.de/

Castles/castle tours:

Mosel Region: Burg Eltz
Rhine Valley: Marksburg

Nothing against Trier but I am not aware of any official "oldest city" of Germany. For sure Trier is as well as Worms, Augsburg and some other places among the oldest settlement - not opening the discussion when a settlement is a town or even a city. But does a Roman outpost matter when when people are interested in history of Germany? For sure it depends on in which part of German history someone is most interested in. In the "newer" parts such as Deutsches Reich, World Wars, Cold War and the time of two German states, ... there are other places which seem to be more relevant for these epochs and topics.

But does a Roman outpost matter when when people are interested in
history of Germany?

Sounds like you might be of the opinion that Trier's Roman past is in fact unimportant. Why not just tell us why that is so, rather than asking a question?

Trier bills itself as Germany's oldest city...

Erfahren Sie mehr über die Geschichte der ältesten Stadt Deutschlands
in den einzelnen Epochen...

https://www.trier.de/kultur-freizeit/geschichte/ueberblick/

I don't particularly care if Trier's promotion people are correct - or what its age ranking is vs. Cologne or Worms - it's a hairsplitting exercise to pin that down, I suppose. I will leave that to others. But I can say with some certainty that Trier's very advanced age makes it an interesting place for New-World North Americans interested in history, which is what the OP said...:

"We like history..."

Yes, pretty broad, for sure. But I'm choosing not to read into that comment any specific restrictions (like from 1871 onward, the history of the modern state of Germany) as you seem to be doing. Please feel free to make suggestions that fit your personal interpretation of the OP's comments.