Creepy protagonist

It's not paranoia if they're actually out to get you.

It's disheartening how many people who claim to love children, and have their best interests at heart, are actually there for the chance to grope them, or worse. That makes people legitimately concerned if someone pays more attention to children than the average adult would. That doesn't mean that all such adults have nefarious attention. Just that people can't ever be 100% sure that the cheery next-door neighbour who loves to have all the kids over to his house, is really doing it from the goodness of his heart.

And I wonder what the rate of unreported molestation is in those countries where everyone is so "free and open" around children.

I agree with the majority here, as long as you explain his predicament as well in the actual story as you have here, it won't seem creepy. In fact, I'm interested in it, I think it sounds like a good read

I know I'm jumping on this band wagon a bit late, but I wanted to add my two cents as someone who has worked with kids for the past ten years.

Unless a child is abnormally shy, has social anxiety, or has been actively conditioned to fear strangers, they are very open to bonding with adults who they can relate to. Obviously, it takes some work on the part of most adults to choose to be a person the child can relate to. You need to be able to express emotions in terms they can understand, show enthusiasm clearly, and be an extremely active listener. You're describing your MC (who I really like the sound of already!) as someone who is at the child's level, so he'll have that more obvious enthusiasm when he's interested in something and more simplistic emotional states, so the child will be able to understand him better than he understand other adults. And let me tell you, an adult that a child can successfully read, no matter how much they are a stranger, will eventually (or quite rapidly) become a comforting presence for this child. Kids can be startlingly perceptive of when an adult is behaving in a way they don't understand, but they can't put that into words easily and it's a scary thing for them. If the kid can understand this grown man, they are going to be best buddies.

In terms of hair ruffling, that's in my mind a completely innocent way of expressing affection for someone shorter than you (who doesn't mind their hair getting messed up and is okay with you touching them). I don't think there's anything wrong with him hugging the kid either.

As for the accusations of the cop being a pedophile, I think that's something you absolutely need to address in the story, because at least in the US (I'm American but recently moved to the UK), men can hardly work with children anymore because people are so paranoid. So yeah, if this guy is hanging around with a little kid, people will say nasty things. But the reader follows the cop for the duration of the story, right? So the reader knows that nothing is up. Honestly, I'd like to see a story that shows a man having a healthy friendship with a kid because it might show people how destructively paranoid we've become.

Long story short, coming at this from the psychology of the kid, there's nothing wrong with the relationship. Actually, I think it could be very good for the child. A kid who was kidnapped, adopted, and sold? He desperately needs someone he can trust, and young boys always need more male figures in their lives anyway, so your MC sounds like a very good candidate for that.

Best of luck!