This is an interesting question. I hope this helps (at least until a better answer comes along!)
I would take him into the bathroom and turn the shower on hot (but don't tell him yet.) As it's warming up, ask him if fire can start in water. If he says yes, step back even further and ask him how fires are put out. (Hopefully he'll say water.)
When he admits that water can't catch fire, show him the steam rising from the hot water, and explain that when water gets warm enough, it actually can free itself from droplets as much smaller droplets, called water vapor/steam. Show him steam coming from the tap, the shower, your hot drinks, etc.
His body actually can give off water vapor, too, even though he's not on fire, but he's warm. Put a spoon in the refrigerator overnight, and have him exhale on it for a few minutes; water should condense on the spoon.
Water can never catch fire. It exists in three states: ice (coldest), water (cold to hot) and gas (water vapor). They are all water. Just as water came from his body, water can escape from hot foods.
If you want to further differentiate steam and smoke, have him blow out a candle and watch the smoke rise. If you hold a spoon over a burning candle (where there's fire, there's smoke), you'll see soot form. Ask him if that looks like water (it doesn't; it's black.) You can then hold a spoon over his hot food, and see what condenses on it. If it's not sooty, it's water/steam.
Good luck; that's the best I can do right now. :)