Did a little bit of math. If you take the lifting capacity of helium and take the volume of a cylinder 20' diameter and 6 feet tall, you get a lifting capacity of around 120 pounds. Subtract the weight of the envelope and the boy and there is still some lifting capacity left. So he could have been lifted by the balloon.
However the balloon looked deflated and I would say about 1/2 the capacity that I calculated when it landed and from the picture right after it took off. This leaves only about 60 pounds of lifting capacity, at sea level. So I don't think it could have lifted off with the kid in it.