I used to have a similar thought at about that age, basically what if I was the only real person, everyone else was just a robot, or illusion or something.
It made sense, I could only see through my own eyes, only feel my own feelings, so how could I possibly prove that anyone else was real.
It wasn't an overwhelming worry, but it did stay with me for a while, and unlike your son I didn't tell anyone, because if I did, they might just poof out of existence.
You'll be pleased to know I turned out (almost) completely normal. My worries these days are entirely mundane, - bills, the nuclear apocalypse etc.
If he keeps bringing it up then I'd just try and have an open conversation with him about it, get him to explain his ideas, poke holes in them where there are issues, but remain open to them. It might actually help him to know that incredibly brainy people like Stephen Hawking suggested that this was a possibility, but that if so then the simulation is so incredibly life like then what's the difference, you're still living your life.