Kerosene in his mouth with a torch near by. That's how they usually do it, or instead of Kerosene they use a high proof alcohol.
KLO
KlogzDeviant
I tried that once. Except with a can of deodorant spray. The flames travelled up the mist and I had to drop the can otherwise it would've exploded. I also had minor burns on my arm becuase the fire travelled down the stick.