Ds2 is 5.8 years. He has little fear and can find it hard to understand cause=consequence. He is a menace in the kitchen, which is open plan to the dining room. Today he was in the kitchen with dh while he was cooking some rice on the hob, ds2 kept leaning on the oven door (he is a pain at opening this) and his head was very close to the boiling water in the saucepan. Dh must have told him at least half a dozen times to move away, be careful etc, that he will scald/burn himself, it will hurt etc. ds2's reply each time was no I won't, it won't hurt (his reply to most things is this, even if he has hurt himself or broken something before doing the action).
Dh brought him to me in the lounge, to try to get back up on what he was telling him, which of course I did but still he wouldn't believe it, so I showed him some pictures on the iPad of scalds and burns. They were not very graphic ones, but ones showing some blistering and scarring. Was I unreasonable? This is an ongoing issue with him. He also has a worrying obsession with fire and will try to blow out the hob when it is on.