As an adult who also sometimes enjoys 9 hr+ simulated flights, I can attest that being able to pause the sim and step away for a few minutes, hours, or days to attend to real-life activities is absolutely essential to my ability to enjoy and even participate in this hobby. Learning methods to pause these experiences so as to resume them later in a satisfactory way is an exercise in both technical and life skills.
Your son probably has understandable reasons for not feeling he can pause the sim. But it is indeed possible even in Microsoft Flight Simulator 2020, if this is the sim your son is using.
I'll back up for a moment to provide some background you may not be familiar with but may be helpful in understanding and managing the situation. There are multiple simulator products on the market of which Microsoft Flight Simulator 2020 is the most popular right now pretty much for the sole reason it can generate an impressive looking out-the-window view of most areas of the world with minimal technical fiddling. Every other simulator product I know of can be easily paused in such a way to allow for a straightforward resumption of the flight at a later time. This is less the case in Microsoft Flight Simulator 2020 due to a case of the product becoming a victim of its own philosophy of being a "digital twin" of the real world. Resulting from this philosophy is a set of features encouraging the user to always operate in current, right-now conditions of weather and air traffic which it is able to simulate based on internet data. Because these features can't be paused, you can't really use them as intended if you pause the sim. This is probably part of your son's concern with pausing.
Another factor may arise if he enjoys using additional add-on products which provide virtual air traffic control provided by real people playing the "game" along with you in real time, many of which feature real-time events with other users he may want to participate in.
So given this, sometimes it may be understandable if he feels the need to complete a long session without interruptions. Is that OK? Maybe so on occasion. Will life always allow him that opportunity? No.
The long-term solution may require him to realize this, and learn work-arounds which allow him to enjoy both flight sim and other real-life opportunities as they present themselves. This might require learning how to sometimes accept the compromises inherent in the available means to pause this sim.
To make things complicated, there are really three different "pause" functions specifically in Microsoft Flight Simulator 2020 which it may be helpful for you to understand:
1. "Active Pause" accessed by a button on the in-flight sim screen or keyboard (Pause key by default I believe): This is the most obvious features but really is not a pause feature at all, so explains why lots of people don't think pausing the sim is possible. It just makes your airplane freeze in one spot in the air while everything else continues (time going by, engine running and making noise, instruments responding and etc.). It's very unnatural, causes problems on resumption, and in my opinion pretty worthless.
2. Pressing Esc from within a flight to enter a "pause" menu screen: This is almost a real pause. This may be settings dependent, but in my installation everything stops including the clock *except* that live weather and traffic are not paused. This means that the weather will have changed between when you pause the sim and resume it later. Not too realistic, but sometimes we have to make compromises in life.
3. A pause feature from within a developer mode menu which takes a little but of fiddling but which can be set up fairly easily if you search for "MSFS 2020 developer mode" for instructions. This definitely pauses everything (except for live weather/traffic) including the clock, except oddly the sound.
Given all this, can your son pause the game? YES! Will he be able to do so in a way that preserves an experience he is trying to have that involves using live weather and live air traffic control? Maybe not all the time. Does he need to use all those features all the time to enjoy the simulator hobby? Absolutely not!
Varying experiences are the "meat" of life and for flight simulation too. Maybe now and then an all-day flight in a multiplayer event with live weather, live ATC, and no interruptions could be a fun and healthy way for him to spend a day. But not necessarily every day. He could still use one of the pause methods above and complete a long flight over several sessions, even over several days, by turning off live weather and not using live ATC services, so to allow simming to fit better with the real world on some occasions. The suggestions by others about opportunities for real-life aviation experiences include a lot of great ideas for keeping his aviation enthusiasm from being a strictly glued-to-the-computer affair. And he might want to try out some of the other current major simulation softwares (Prepar3D, X-Plane, and AeroFly) which have been overshadowed of late in popularity by Microsoft Flight Simulator 2020 among casual simmers because they are less visually impressive, at least without extra work and fiddling. But they each still have their own strengths compared to Microsoft Flight Simulator 2020 and have lots to offer for an aspiring pilot looking for more "professional" and less "game" oriented experiences. And all these sims can be fully paused at will.
