For me personally, the joy of riding a bicycle is in the street and path I'm on in the moment, and not the things around. Like the freedom you get sitting on a motorcycle headed along a nice country road.
Optional: To encourage that feeling from the very beginning, it helps if the slower person keeps up on an E-bike. I love to be able to go as fast as I am, while the other person just adjusts the E-bike help level. On an E-bike, the achievable routes for a beginner are also interesting from the start.
A nice beginner downhill ride gets you excited about gaining more stamina to get higher uphill first. (He should feel no shame about stepping down off the bicycle, walking it slowly up, and riding it down, if he finds a track too steep.)
Because you wrote that he bought a mountain bike, I think he wishes for experiences like these, and it will be hard to stay motivated on a flat surface.
Another factor is how easily he can transport the bicycle to an interesting track. (I have a two-bike holder you can fit onto the tow coupling of your car. This is easier than putting the bike on the roof or inside the car.)
As a newbie myself without friends interested in mountain biking, a phone holder for navigation is great to find new and interesting routes easily.
For extra motivation, put mountain bike magazines beside the toilet, to visualize what's possible when his skill level grows.
To get fit because of numbers or because of an app never motivated me personally. Also, I don't like to do conventional 'training' or to have fixed schedules. I just like riding my bicycle.
So, in conclusion, find out why he drives the bicycle and motivate him with a tailored plan.
When he was 15, did hobbies such as swimming motivate him to ride his bicycle there, or the feeling on the bicycle itself?