Went squirrel and rabbit hunting with my Dad starting at about age 5. Hunting teaches very quickly about the effects on being on the wrong side of the rifle when fired. I was allowed to shoot my Dad's rifle which was an old (even in the 50's) Winchester pump.
Got my first BB gun when I was six and shot a lot of birds and cans. I already knew safety and the mechanics of sighting from hunting.
We had a single shot 22 in the family. It was originally my mother's and then got passed to my older brother, then me when I was 8, and finally to my younger brother. Best shooting 22 I every owned with iron sights.
Daddy's eyes finally forced him to buy a bolt with a scope and I inherited the pump.
I have had 3 AD's in my life. Although scary, none was dangerous as the barrel was always pointed in a safe direction...even when empty.
I taught my wife to hunt just before we got married. Only way she could spend time with me during hunting season. I found that I had to really think hard when training her to handle a weapon correctly before seeing the mistakes she made. Gun handling is so instilled in me that I don't consciously think about it.
One key element in learning to handle weapons correctly is experience. The more your son handles the weapons, the safer he will become to the point it is second nature.