I am not saying it is his problem, but it can backfire by just turning on the key. I have seen it many times on propane fueled engines. A slow leak of fuel into the cylinders or intake and just at the right time you get a spark from the coil when you turn the key on. I can't see it happening everytime, but once in a while is very possible. As stated before you need to get your idle down to normal. Turning the car off at high idle may allow some gas vapor to sit in the cylinders, and then the next time the key is turned on, a spark is igniting the left over fuel. Maybe.

James