Day trip from Paris to see castle(s)?

Would seeing Chambord or Chenonceau be too similar to Versailles?

No. The exteriors of both are the highlights not so much the interiors. Though the staircase in Chambord is unique.
I did my own day trip down there but here is list of tour companies ranked by trip advisor
https://www.tripadvisor.com/Attractions-g187196-Activities-c42-Loire_Valley.html

Most of the chateaux in the Loire are palaces, not castles, in the strictest sense. If he's interested in a royal fortress type of castle, then he could visit the Chateau de Vincennes, which is a short ride on the Metro or RER.

We visited both Chenonceau and Chambord in one day, although we were staying in nearby Amboise, and riding bicycles we’d rented in Amboise. They are different from Versailles (where we also rented bikes, to ride around the grounds), but all are fancy mansions, not really defensive forts. The exterior gardens are great to see, in addition to the interesting interiors. One of the things I remember most about Chenonceau is the ballroom that straddles the river, and while still sumptuous, the floor is uneven from age and settling, so no one dancing in there now would be gliding effortlessly across it.

For medieval castles, you’d want Cahors, Carcassone and the Languedoc, Beynac and the Dordogne, or Josselin in Brittany to start with, all farther from Paris than the Loire region.

Sorry but the best defensive castles are in England and Wales.

There are several very viable defensive fortifications within easy reach of Paris. The extensive Vauban fort of Metz is much diminished, but the remains can easily be seen, walked, and even stayed in. For a medieval castle you can point to Homburg, in Germany, or Vianden, Luxembourg, both of which have been rebuilt/maintained and are worth visiting.

https://sites-vauban.org/en/node/7801

https://www.homburger-schlossberghoehlen.de/

https://castle-vianden.lu/

Does your son speak passable French? If so he might appreciate a guided tour (in French, at least when I was there, although that was quite long ago) of the Chateau de Vincennes. Although the Chateau has suffered quite a bit of destruction over the years, it is still possible to see how it was once a mighty fortification. Peter

Some of the base of the medieval Louvre can be visited under the Louvre.
Loches in the Loire Valley is the remains of a defensive castle, as is Vincennes mentioned by others above.
A couple of today's streets in Paris are what remain of the paths that circled inside and outside some 15th C city walls: rue d'Aboukir and rue de Cléry.

The ruins of Château Gaillard are close. I have only driven by them, so I cannot speak to whether it is worth making a trip just to go there.

Thanks everyone! Very helpful!

Angers is a classic fortified castle in the Loire Valley— 90 train minutes from Paris.

The Conciergerie in central Paris is the fortified portion of the palace-turned prison that held Marie Antoinette after her arrest during the Revolution.

Angers is also stunningly unique with black and white striped battlements. The magnifcent Apocalypse Tapestry is also displayed there. This unlike the Bayeux Tapestry IS a tapestry -- huge -- maybe 100 separate tapestries telling the story of the apocalypse -- very much worth the trip.

But Chambord and Chemonceau are nothing like Versailles and as other noted, it is the outsides of each that are amazing. The Loire castles are as noted however palaces not fortresses. for that the one in Paris Chateau Vincennes actually does very nicely and is on the metro.

To my opinion the most interesting castle to visit is Château Guédelon. It’s a construction site of a fortified castle built the medieval way from scratch, so using the technology and materials of that period. It will be finished within a few years but still a lot of work needs to be done. The site is located in the middle of nowhere in Burgundy, so getting there will be a real challenge and time consuming. It’s unique, but worth the effort if you ask me.

https://www.guedelon.fr/en/

As already noticed most castles are turned into palaces. Besides the already mentioned fortified castles you can go to Ghent in Belgium for it’s Castle of the Counts. Ghent is anyway worth a daytrip and getting there is pretty straightforward. And famous for another thing from medieval times: The Altarpiece in St Bavo’s Cathedral. You can also visit Lille between changing trains there.

https://historischehuizen.stad.gent/en/castle-counts

Never got inside of the following, but Caen in Normandy has a huge fortress, Château de la Madeleine in Chevreuse and Château Dourdan, both south of Paris are fortresses too. Last but not least Château Fort de Sedan (visited 20 years ago) near the Belgian border, one of the biggest in France. Having a car well to combine with the one in Bouillon just across the border in Belgium.

https://www.chateau-fort-sedan.fr/en/
https://www.chateaudebouillon.com/en

Dourdon is easy to reach by public transport - we did it last year and the stroll along the river for the decorated wash houses is fun on a nice day. The fortified castle is however not a much different experience than Vincennes and of course for much more effort. to get there.

Thanks for the heads up Janet. I have removed that sentence in the meanwhile. I have discovered Dourdan driving through the regional natural park of the Haute Chevreuse Valley. A surprisingly charming region so close at the southside of Paris that is certainly worth exploring, but a car is needed for this.