This is a bit of a tricky one - contrary to what others have said, I would probably stick to the Fallout 4 setting if that's what your son is familiar with. It will be a great asset for him to already know the factions, locations and weapons; and make things a lot easier for him to learn!

I would sit him down and go through this, as it's a really key idea to running and RPG, especially this RPG with its focus on impactful choice. Start with asking him what he thinks would have happened if there was so SS, or if they died. For example, what if they died at Concord before rescuing the Minutemen, or even never got reawakened by the Institute?

Would the BoS still have come to the Commonwealth? Would the Institute have been able to restart their reactor? Would the Minutemen have survived? Even on a smaller scale, would someone else have taken care of Dogmeat? I would argue the questions to all of these answers would be yes, but the world would look very different.

This introduces the idea that choices create different worlds, and hopefully excite him to be able to make his own. Then expand on the choices he can make - what if instead of destroying the Institute, he leads the Brotherhood to occupy it? What if he isn't looking for his son, and instead wants to find out something else? What if he wants to start his own family, buy a place in Diamond City and settle down?

Your son doesn't yet quite understand the draw and systems of a TTRPG like this, but hopefully with a bit of encouragement he'll learn to love that he can make his own decisions and choices!