Nitro isn't going to matter. Something is wrong if he's burning that much fuel. Even an 8 ounce tank should be able to give him 10 minutes in a trainer unless he's staying at full throttle all the time.

You said the tank is empty, but have you verified that? Can you actually see that there is no fuel in the tank after the plane deadsticks or when you land because you think it's almost empty? And have you checked the fuel compartment for leaks?