Man, some people! That first owner is a bloody idiot, plain and simple; the proper thing for him to do would be to apologize to you and your son for what he allowed his dog to do, not blame either of you for your completely rational reaction to the situation.
The second dog owner got it right.
In addition to this complaint, here's from the opposite side of the spectrum; having someone approach your dog and try to pet it without first consulting you. I had this happen, and my dog (who has been dead for nearly six years, at this point) reacted defensively, almost biting the person in question.
A grown man, by the way; just walking by, didn't even say hi first or anything. If he had, I would have reined her in, and let him know she was very protective of me, especially when it came to men (I actually have no idea why that was; I'd had her from a pup).
Said man had the nerve to look at me like I was in the wrong. Luckily, my dog adored kids. Again another mystery to me, but never was she more gentle, than when children were around.
All this being said? I never let her run off leash in public areas; no matter how well-mannered a dog may or may not be, it's better for the owner not to take chances; both for the sake of anyone who might get hurt by the animal, and for the animal, itself.