I handed the boy a 100lb weight to play with in the lake
Carrying that much weight would make him drown, so why did you do it?
Don't worry. He can swim with it.
You shouldn't weigh someone down if they're trying to swim in a body of water. They could end up in a bad situation or even drown due to the extra weight. You'd be responsible for hurting this person due to you giving them the weight.
i am just trying to teach them to swim under stress.
This is a bad idea to weigh down a kid with a huge weight to teach them to swim better. If you want to help them strengthen their body to be a better swimmer, then workout with them in the gym on dry land.