Where else to go?

A big help is what "too much travel" means.

And is he limited to by car, train, or plane? IOW, you can easily drive to Germany, Belgium, France, or similar in car. A little farther if by train (and choosing night trains, too). And, flying, of course gets you all over.

Honeymoon? Take the high speed train to Paris.

You only talk about your son and his interests. So I can’t help but wonder if the future wife has any say in this. Or is the honeymoon destination a surprise for her? Even then I would take her interests into consideration too. After all, happy wife means happy life….

Paris is a great suggestion.
If they want something that is not another city, they can head towards the German Middle Rhine and fly home from Frankfurt. The train journey from Amsterdam is relatively easy. First stop could be Cologne, then work their way towards Frankfurt visiting castles, half-timbered villages, vineyards etc.

Amsterdam is a world capitol and while it has uniquely Dutch characteristics, I would suggest spending some nights in real historic Dutch towns.

There are literally many dozens to choose from but the ones that I've been to that I love are Delft, Hague, Leiden, Haarlem, Hoorn, Zutphen, Middelburg, Utrecht, Gouda, Maastricht. Heck, that's just off the top of my head; there are so many.

Since this is their honeymoon, will they be traveling at a slower pace? The Netherlands has loads of postcard towns that are perfect for strolling. Last May/June, my husband and I were in Hoorn and loved the town! The biggest highlight was just walking the streets and through the parks. Enkhuizen is an easy train ride from there, and it has that same lovely feel.

Also, what time of year are they going? The weather will dictate whether they want to visit the strollable towns I mentioned earlier or stick to places with more museums and indoor activities. If the latter is the case, Leiden is quite enjoyable. Utrecht is another good option. Both cities offer good rail connections to other towns for day trips.