P.,
I think a lot of kids are confused about the subject because our society teaches comflicting and confusing messages about stranger danger. We say "don't talk to strangers," but we expect our children to speak when spoken to by a new doctor or cashier. We say "don't take things from strangers," but we let them have samples at the grocery store and suckers from the bank teller. No wonder they are confused.
I have told my children a little differently and I hope this will at least keep them a little safer.
1. Don't talk to a stranger without me there. If you want to pet a puppy of someone who brought their's to the park, you ask me to come with you. Don't go by yourself. If you are at school and your teacher introduces you to someone, it is safe to talk to them WITH your teacher.
2. Ask my permission if someone offers to give you something.
3. If you ever get lost, find someone who wears a badge or works where we are. We have spent A LOT of time learning how to spot employees at the store, the library, the mall. I teach my kids to give the employee their name, my name and my cell phone number. If this is too much info for your son to remember under stress, keep a card with the info on him - in his pocket or around his neck.
4. If someone tries to take you away or hurt you, you have the right to do ANYTHING to them. You can scream, hit, kick, bite, scratch, ANYTHING. We also roleplay situations so they can practice knowing what to do. (No, they can't hurt anyone, just practice talking, decision making and running.)
5. Until he truly gets it, keep him well supervised by an adult whenever you are not at home. Make sure that your son's school has good supervision on the playground.
I saw a Dr. Phil experiment where they took a group of kids (with parental permission) who could give all the right answers about stranger danger. Then they put them in the park and had a paid actor come up to the kids and ask "do you want to see my new puppies? They are in my car." EVERY SINGLE ONE OF THOSE KIDS GOT IN THE CAR!!!!! And they didn't get alarmed when there weren't any puppies and the actor shut the car door with them in it.
I think that kids envision "bad people" as the kind they see in stories and movies. Dark clothes, mean looking, scary music. That is how they spot a bad person. The real evil people know how to blend in. I talked to my kids about that a lot, too. I used the White Witch in the Chronicles of Narnia as a good example. So nice at first, cool reindeer and sled, pretty white dress, shared her Turkish Delight. You can't judge people by their looks.
Last of all, I think we need to use caution not to scare our kids too much. We don't want them to live their lives constantly worried that they are going to get kidnapped at any moment. It is so rare. Statistically, stranger abduction HAS NOT SIGNIFICANTLY RISEN in the last several decades, despite the great increase in media attention. Well over 90% of abductions are done by family members - usually in some kind of custody battle. That is a whole other can of worms, but since you are married, the chance of your children being abducted goes way, way down. It doesn't mean that you shouldn't use common sense, but that fear shouldn't govern your life either.
Hope this helps,
S.