Quick and dirty math post-
The media reports the balloon was about 20' x 5'. Modeled in SketchUp as a 20' diameter sphere, shmooshed to a disc with a 5' vertical, that yields ~1000 cu. ft. of helium, or ~28,300L. As we all know, helium can lift 1.113g/L, for a total of about 31.5 kg of lift for that volume of gas. A quick Google search tells me the average 6-year-old boy weighs 46.2 pounds, or 21kg. That leaves 10.5 kg for the mass of the craft itself- about 23 pounds, or slightly less than the weight of three gallons of milk.
I guess it's plausible, but I'm skeptical. I think it will come out that he was never on board in the first place.