He is using the force

Sometimes I forget what a powerful bender that kid is.

He might have slowed it down a bit, we don't know

That's what thoughts and prayers are for.

He's trying to,stop,it with the power of his mind. Now, imagine if the combination of air temperature, distance from the volcanoe, etc., caused the lava to,cool,enough to stop. He could play that off to people who think he really does have power.