It is very frightening that your ex is so ignorant of how predators work. He needs a serious (though figurative) slap in the face. I would absolutely want to insist that he get some parenting classes where the techniques of predators are frankly discussed. He needs someone to scare the heck out of him.
It is equally frightening that he's apparently just fine with the idea that "this must be someone who knows our son." Shouldn't mom and/or dad know who the adults are who know their kids?
It's also worrying that your ex simply does not believe your son when son says he didn't know this man. You do realize that your ex is, basically, denying what your son is saying when son says he doesn't know this person? Dad is not trusting son, and it was your son who had the gut instinct and the good sense not to approach this stranger. Dad could use some good sense but is utterly blind and uninformed.
Well, it sounds like dad is not going to grow up, or get the lessons he needs, so praise your son like crazy for being smart, and emphasize with your smart son that he has witnessed the real world where there ARE predators, and that things could have turned out very differently. If he's elsewhere and there's no camp counselor there to handle it -- he needs to stay smart and walk away. Ensure he knows about things like sticking with moms with kids when in a crowd; not being afraid to yell and scream if someone tries to grab him; being aware of his surroundings; going quickly into the pool area where the lifeguards are, etc.
BUT -- I can see why the camp did not grab the phone and call parents to tell you about something that actually never went any further in that moment. It is possible that the person was there to pick up a kid he didn't know well but was legitimately allowed to pick up? It would think not, since it was at a pool (unless the pool is within a camp property); it sounds like the kids were at a public pool and this guy was trolling the pool...But you still need to get the details from the camp director; was this a case of a random stranger driving past or an adult who wasn't familiar with the pickup procedures and the kid he was picking up? I work at a summer camp and there have been cases of adults who were third or fourth on a kid's list of "authorized pickup adults" turning up all confused and the kid saying, "Wait, I had no idea X was going to get me" and so on. I do agree, the situation you describe sounds very odd and like a random person, but that may not be the case. A lot depends on whether this was a public area and not within a camp.
I'd talk to the camp director and calmly, professionally describe what your son said and ask if the case was a confused but legitimate person, or what happened. Be SURE to praise that camp staffer for going up to that driver and asking the right questions (like last name ID) and sending the person away! Then ask what the formal procedure is when this happens. I would ask whether the camp staffer who talked to the man reported the incident higher up, to the director. I would probably tell the director that it would be a smart policy if the camp required counselors to report any such approaches immediately and if parents were notified so they could let the camp know if possibly a pickup just went awry. And I'd want to know so I could comfort my kid with the fact that the camp staff is all aware and watching for this kind of thing.
After talking to the director, if this is a case of a stranger trolling, absolutely report it to police. It's too bad you didn't know about it the day it happened, but still report it. Police build lists of such reports and your report could help them build a picture of a person they are already watching out for.