I don't know anything about an atv,but I have run diesel equipment all my life,and it sounds like the air/fuel governer is stuck or out of adjustment.When a turbo goes out,they will do the same thing as you describe,lots of smoke and no power.
I had the pin fall out of the fuel rack,on a cat 992 loader and you could not give it fuel,and the engine would not rev up.
Sounds like his is starving for fuel,whatever the cause.
Just my .02,
992