This is one of those things where I think supervision is key. If he is on the cognitive level of an 8 or 9 year old it may not be in his best interest to leave him alone long enough to light hundreds of matches or start a fire in the back yard. I kow how difficult it can be to keep an eye always on an ASD kid, but we've learned to be a different kind of parent when our children have special needs and their safety must come first. If he is not at the cognitive age to understand danger, make sure he is never left alone where he can put himself in a harmful situation, jut like you wouldn't leave a young child in the kitchen with something on the stove. I imagine this is going to require a lot of patience. I wish you the best of luck and support in figuring out the best course of action for your ASD child.

My son understands the danger of fire but still just is fascinated by it. I hid my matches.

Can you purchase a fire pit with a safety lid and teach him that there is proper time and place for fires? Maybe every Saturday evening you could have a fire in the pit as a family activity. But if it's not that time or place, no fire.