France at 18 years old?

FUN! Paris - I recommend booking a Fat Tire Tour skip the line tour for the catacombs soon and go to a cemetery as well. I booked a date in July about a month ago and there were only 2 options left to book - $171 for 2 tickets.

Your project seems to spend a vast amount of time sitting on trains. Since he likes travel, he can get to other places without you, as he grows older. While you give good reason for flying to Germany, it makes this trip about you, not about him.

You don't say if you have been to Belgium before. But I'd note that Antwerp might be more interesting for an 18-year old than Gent, and there is much more to do there. Did you omit Bruges for some reason? Does he like Craft Beer? Is he a sensible drinker? If you are in Belgium, Cologne is quite accessible, and has Roman ruins and a cool city center. Because your schedule is flexible, I suggest trying to hit Dusseldorf for their once monthly "Fish Market", which is an outdoor dining and wine festival on the riverbank. (Assuming it survived the Pandemic.)

When you wrote that he "He wants to see several places", is that a list he made, or a general thought? If the former, what is his list? Did you mean to say that he wants to hit several countries? I mean, there is plenty to see in France, especially in the summer. It would help to know more about what he does in his spare time, and what activities draw his attention after school and on weekends. Does he like museums? Museums other than art museums?

Nick, I am confused by your comment: "And too many Americans think that Frankfurt is the only gateway to Germany or that part of Europe - maybe because their uncle was in the US Army on a NATO base or something (??)" Perhaps you can explain. The quip about the Army and implied US traveler stupidity is offensive.

My only non-stop options to Germany from ORD are Frankfurt or Munich. The OP noted similar restrictions. I would call one or the other the only gateways.

Don't miss one of the fabulous virtual reality experiences now on offer. Eternelle Notre Dame is a 45 minute walk through Paris in the 12th century as Notre Dame is being built. One location is right at the Cathedral in the underground complex. The other amazing experience -- and I don't know how long this will be avaialble -- is the Impressionist VR experience at the Musee d'Orsay. Another 45 minute stroll -- this time through the 'salons' of Paris as well as several sites where the impressionists did their work. The highlight for me was watching Monet painting Impression Sunrrise as we stood there in the beautiful scene ourselves.

I think this would be especially thrilling as a way to be introduced to this art or history for a young person.

Took my 20 year old son to Europe. While we did plenty of art museums, I think his favorites were history museums, science museums, and gardens. Wish we had saved more time for gardens and grounds at Versailles. We only had one day at Versailles but needed 2 to see everything. Found a cute campsite called Hutopia in Versailles I would love to check out next time. Exploring the secret passageways of Lyon sounds like a great adventure. We also thoroughly enjoyed Disneyland Paris. It is very multicultural. The shows we saw were a mix of French and English. If he has a particular interest - WWII, Napoleon, Joan of Arc, cheese - whatever - consider making that the focus of the trip.

The bike tour is a good idea, you can do this on your own without Fat Tire company which is very expensive for a group tour. I have paid the same amount for a private guide renting the bikes and helmets for us.
Also, catacombs with group tours is scary and catastrophic for a clostrophobic. You should do it with a private guide or on your own.
I would go with my son to the same places and take the same pictures, same poses same background, just for the fun of posterity.
I would stay in young but safe areas such as la Sorbonne area, north of le Marais but definitely avoid Chatelet and the center center of Paris.
That way he will find friends to hang with and to talk to while having a moment in a café terrace.
If art museums are not his style, then you can't go to Versailles, Orsay, Pompidou, Louvre? Although it is a shame not to go there while you are in Paris, you can try to convince him of the importance of Art History and hire a quality private guide to make it more fun for him ?
You can enjoy other types of museums (science, architecture, fashion...) or tastings together, long walks in Tuileries or Luxembourg.
But personally speaking, think that this age was about meeting new friends. So the staying area is important : safe and student areas.
Let me know if you have more questions.

I think it's wonderful that you want to share some time in Europe with your son before he meets up with his grandparents. But you don't say how much time you have. That would certainly impact any itinerary planning. Have you asked him what he would like to see besides Paris?