Doesn't matter what he thought or was willing, what actually happened was that he hit on someone above the age of consent, the facts of what happened do not warrant sending the guy to jail, sorry to say it but you'll have to wait until you have actual evidence of him doing bad things with an actual minor before taking his rights away. Or at least you should wait until then if your country's legal system is worth anything.

The age of consent in my country is 16. Just some food for thought. I even think the watch list is too far. Depending on the content of the messages to the 18 year old, the most that this warrants is an investigation into the guy.