Make sure you ask him what his definition of a "stranger" is. My youngest dd was like this also. It took me a very long time to get her to realize that anyone she didn't know was a stranger. After learning about "stranger danger" at Pre-K she thought that strangers were horrible monsters or even anliens from outter space.. I know it sounds funny
but she couldn't imagine anyone that looked like everyone else could do any harm to another person.
I found that I finally told her that anyone that I hadn't told her she can trust (even if we have talked to them before) is a stranger. I told her the same things can happen to her that were told with 'stranger danger' like she can be taken away and hurt.
I am not sure that this will help you but remember to ask casually "Can you point out a stranger to me" when you are out at the store or the park or where ever you are. You might be surprised by the answer even if you are the one that who has told him what a stranger is.