Ever adjust your FUEL SCREW?
You must gain access by removing the tamper-resistant cap, lower front of carb.
Use small flat-blade screwdriver; bottom screw gently, then back out 1.75 turns or so. This procedure fuel-enriches the idle mixture from typical factory settings.
Then again, you may be suffering the dread pox of ethanol fuel contamination, partially blocking your pilot jet . . . a "good scrubbin'" is the only way I know to solve definitely this problem, although some praise heavily fuel additives (e.g., "Sea Foam").
All that said, your bike oughta start when cold by application of the "choke" (starting enricher). When ON, a plunger is withdrawn from the carb, opening a fuel orifice and an air passage, insuring a fuel-rich mixture; make sure your choke's working correctly.
If he's at 5,500ft in altitude he does not need to enrichen the mixture.
Quite the opposite, the mixture needs to be leaned out to accommodate the
proper stoichiometric ratio.
Although, the cooler air would increase the ambient air density but probably not enough to make a huge difference and offset the altitude.
Moisture and Humidity will also displace air molecules and require a leaner mixture.
In other words, high altitude and a hot, humid day would be the worst case scenario and would require leaning (less fuel) the mixture to get the best performance.
Low altitude, cool weather and dry air will get you the best possible performance and could require you to enrichen (more fuel) the mixture.
Try starting without the choke and see what happens.
Although, having said that, it seems that it starts with the choke on and dies with it off.
You could try turning up the idle and see if that keeps it running.
Check the air filter as well and make sure its clean..on that note, take the air box side cover off and note weather its better or worse.