YTA. It's ok to be worried about your son and I personally think that the life experience/stages alone is so incredibly different between a highschooler and someone who is 25, but seeing as how he's 18, it's not inherently predatory.

The way you went about it was entirely not ok. He is 18. He's legally an adult. With that, some sense of autonomy or agency should be instilled within him. Completely shutting him down and not even trying to listen to him isn't good for him or your relationship with him. You didn't even try to understand or communicate with him.

If you sat down and actually spoke with him, maybe you'd learn why he thought he needed to sneak around (spoiler alert, it's probably because he doesn't trust you enough to confide in you because, hearing how you handled this, you probably never listen to him.) Just defaulting to "I'm going to kick you out" is also just really terrible. I can never understand that. Just have a conversation with your adult son. YTA.