Me, I'd get him alone one day and say
"Hey kid, I'm not your father and will never pretend to be. But as a man, 2 things I can tell you, 1. You need to speak your mind and 2. You need to admit when you make a mistake. Someone tried to break into somehting of mine, and I'm not saying it was you, but if it was or you know who did, I'd ask you to admit it now. I'm not going to tell your mom, this is all between you and me. If you say it wasn't you and you no nothing, we'll leave it at that. But remember, one of the only only things you control in life is your reputation"
As he's only 18, odds are you can tell if he is lying.
just my thoughts
btw-have 2 kids