The first thing I would do is drop the attitude referring to the victim as "victim" and acting like you did not assault him.

You were playing with something flammable in an attempt to scare him and injured him as a result of of your recklessness. Since, by your own admission you were attempting to scare him, I would drop the "this was just a stupid joke" thing. You were being a huge **** intentionally attempting to cause the victim to become afraid and injured them.

You are very likely to have some level of criminal liability for your actions. If you are only 14, inform your parents, and follow the advice of any attorney that is appointed if you are charged, this is pretty likely not to have any long term negative consequences since it sounds like the kid was not severely injured.

For future reference:

Intentionally scaring people with the threat of physical danger is not generally funny. Unless someone specifically opts in, I would never do this again.

Intentionally scaring people with the threat of physical danger is never funny and the danger is real because you are not in control of what you are doing. You can ruin your life and someone else's with one "stupid" moment when you turn personal safety into a joke. You are too old to need to be told this. You are not a six year old boy.