"Must See" is relative, may not be the same for everyone.
Searching online or in a travel book will show many different lists of "Top Ten" or "Top Twenty" thing to see and do in Paris, but ultimately, it's what interest you (i.e., museums, landmarks, neighborhoods, etc.)
As an old hand traveling with 14 year olds, the best advice I can give is to turn over at least one day to him, give him a guide book, point him at the internet and be amazed at the cool thing he will come up with. We have seen all sorts of things we would not have discovered without our kids research and choices.
I would think climbing the Notre Dame Tower might be a priority and with the new reservation system (you do it the morning of the day you climb) you avoid the long lines that used to be the norm.
Let day one be easy given jet lag, especially with a teen ager. JanetTravels has a great piece of advice re handing a day over to your son:)
There are so many iconic sites in Paris: pick one to discover. Three days is the blink of an eye.
Some young ones (and some not so young) like to visit the Catacombes. I've never understood it myself, but ...
The discussion in the link below, especially reply #3, is aimed to teenagers, but I daresay that some or most would be of interest to adults.
take your kid and do the Sewers tour. What kid doesnt like the pooper?
Definitely hand the 14 year old a travel guide and ask what they are interested in. At that age, my son loved the behind the scenes tour of the Eiffel Tower, going to Galeries Lafayette (he bought himself a watch and some legos), the Dali museum and visiting Patisseries to sample pastries and hot chocolate. He feels the best hot chocolate in all of Paris is at Un Dimanche a Paris, which is not far from your hotel. It's pricey but worth it for an afternoon treat. He enjoyed a game of finding all the dogs in the art at the Louvre (there's a ton of dog statues). Every kid is different - mine did *not* want to do the catacombs or sewer tour. But he did enjoy the military museum at Invalides.