Please pardon my ignorance, but is this 6,000 ft altitude the height above the ground? If so, then I’d guess we’re talking about the oxygen altitude equivalent of ... what — maybe 10-11K given the area? That’s still breathable, but less so for a panicked little boy.
No they will use the actual altitude above sea level.
The cruise altitude appears to be stabilized due to the equilibrium of the object and it's the craft's weight and cargo, versus the lift capacity of the helium onboard.
The craft is now near Brighton, Colo. My guess is that if the craft doesn't descend on it's own as the helium escapes through the craft's outer surface incrementally, there is the possibility of impact with a mountain range... however, the winds tend to scream up over the top of ridge lines, turning into waves or rotors on the down wind side, which could carry the object over the ridge or peak, and on to further adventures. The setting set will impact the performance, as the temperatures plummet. This may be what brings the craft down in the end.
Hang on tight little big man.
He would feel lethargic from the oxygen level, maybe pass out.
Plenty of oxygen