1 liter of helium can lift 1 gram. That means a 20 foot diameter spherical balloon can lift 263 lbs with helium. This was not a spherical balloon, It was saucer shaped which dropped it by about 2/3rds roughly in volume, giving in roughly 1400 cubic feet volume of helium, giving it a lift of about 88 lbs at seal level. Denver is about a mile above sea level, and since altitude decreases lift in respect to a balloon with helium, its lift capacity drops as well.
The kid was reported to be about 40 lbs, and along with the actual balloon material and canopy weight, it is impossible for a balloon of this size to be able to exert enough lift.
20’ diam => 3.14*20^2 = 1257 sqft
5’ ht => 1257*5 = 6283 cu ft *28.32 L/cuft = 177940 L
(It’s a cylinder, essentially.)
2.205 lb/kg
If 1 L lifts 1 gram, then
(177940 L * 1g * 1kg * 2.205 lb)/(1 L * 1000g * 1kg) = 392.36 lbs
Now, what the structure weighs, I don’t know; but if the man wanted it to float, it’d better weigh alot less than 392 lb!
And BTW by these factors, a 20’ sphere could lift 2093 lbs.