This is a lesson I learned from my mother. When my siblings and I were this age, we were given responsibility to take the garbage out back and to set it on fire and make sure it got burned (we lived on a farm and this was completely legal at the time and how everyone on farms got rid of their garbage). The idea was that of course we were curious about fire. The setting allowed us to learn about fire, know how to deal with fire, and possibly yes, burn our fingers a bit.
I taught my daughter how lighters, matches, and candles work when she was 8. I allowed her, under my supervision, to light things on fire and see how they burn. I would never have thought to keep these things secret, or hidden, or forbidden at that age. Children are curious about things like fire, and need to know about them. A child who knows nothing about fire could panic if they accidently start something on fire (lighter, stove, candle, etc.). I would much rather be part of educating my daughter about fire than prevent her from experiencing it.
I don't think your son needs to be punished at all, just educated.