I think you’re right to caution some prudence here. Who knows what the kid meant?
Somewhere I read where the two older boys were up on their roof video taping the ballon flying away and told a neighbor their kid brother was on it.
I can almost see the older brothers daring their kid brother to untie it so they can film it, and the kid brother afterwards is afraid of what the old man will do to him so he hides in the attic so as not to get caught.
Although the dad almost sounds loony enough to be behind the entire thing too.