Liter of helium lifts about 1 gram. Not knowing the exact dimensions of the ball and the weight of the kid it's hard to say if it could have lifted him to air.
But if we presume the ball was a cube and 20 by 5 by 5 feet as stated in the article, it should be able to lift about 13,5 kg.
e. So it was a dome, well, someone else can do the math for that.