There's no way that balloon could've lifted a 50 lb boy
badcheese.com
badcheese.com
Also based on how it flew and the fact that, given the authorities knowing the size of the balloon, there was no shifting of the bottom section when it rotated. I'd imagine staying on that piece of plywood in the same spot would be just as difficult as staying on a balance board, so they pretty quickly should have determined the balloon was not a worthwhile search topic.
Things just didn't add up and now we obviously know why.
The father should have known roughly how much it could lift, and I think I saw a statement that it could only life its battery pack. But he was probably willing to cast that evidence aside for the (erroneous) eyewitness testimony that it happened.