Isn't it illegal to lauch a balloon of this nature?

Also, there seems to be a degree of acting in their alarm that their son may be in the balloon. Note that they did not verify the concept, only started reacting to it.

The family knew the boy wasn't in the balloon.

The father built it, he had to know how much it could lift.

Anyway, I went ahead and ran the numbers.

For the balloon to get off the ground with the boy in it, it would have had to hold 45,360 liters of He, or the equivalent of about 46 four foot weather balloons.