I agree that it's not a mistake and that it's a good idea for you to learn more about it and then explain it to your son in terms he can understand.
The most important thing, I think, is for him to understand that sleep paralysis is a temporary condition. Sometimes when people are waking up, they can't move or talk. The problem can also happen when drifting off to sleep, but it's more common when waking up. It goes away quickly.
It's very important to understand that people don't go to sleep and become permanently paralyzed for mysterious reasons.
