My son is 10 and is very skinny.

I have a Laser on a mid-size lake and let him take it for a spin.
Amazingly, he was running up and down like a pro. I know that they
make a smaller sail, but he still can't upright the boat by himself.
How do the kids in these Junior regattas (I read about them but never
seen one) manage? Is there a weight limit.

A 10 year old should be able to right a laser. Be sure he lets the main loose
(should happen as it should not be cleated) and stands outboard on the dagger.