I have had some accidents with oil spills, but they weren't burnt. How long does it stay in the ground if it is not burnt? Also, even if he burnt it, some of it still spread into the ground, so perhaps only a little actually got burnt.
"Organic" requires a three year certification that no chemicals were used. The reality is that if he used it lightly and poured it on the plants and not in the soil, 99+% was consumed when he burned it. Yeah, if you want to scrape off the top inch, fine. Gas has no lead anymore, up to 10% is corn alcohol, and the soil has natural hydrocarbons. About the worst thing is he may have toasted some earthworms. If it had been xylene or other chemicals, I'd be more concerned.
You do need to slap him up the side of the head though. Gasoline is NOT a real safe thing to being burning in an open flame, especially on hot days when the vapor can be more dense.
Within minutes of being brought home most cats can train humans to open tuna cans.