Unless your conversions are wrong or I missed something, assuming a cylinder of 20x5, this thing filled with He can lift 392 lbs.
Alot more than 40.
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.
When you figure the weight of air versus helium per cubic foot That's hardly enough buoyancy to lift the boy at sea level even if you don't include the twenty five pounds or so that the balloon envelope, and gondola must weigh. Then when you figure in the fact that the air is down to half of sea level pressure by 18,000 feet you'll see that that thing could not have lifted off from Fort Collins 5000 feet above sea level with a fifty pound boy on board let alone soared to 15,000 feet as reported.
6m diameter, plan area c28m2 mean thickness maybe 1¼m = total volume 35 m3.
Helium lift 1Kg/m3 35Kg =c.80lb at sea level. Reduce by 25% to allow for altitude
Gross lift balloon and payload=60lb (best mental estimate, but would be surprised if out by more than 10lb either way)