Wow, Morty! Are you in for an adventure!
My own are 22, 21, and 9, and my borrowed one is 17, but he is different. And how.
It sounds like you have all the tools. Mostly use your intuition. Some of them are verbal and some, like my eldest, are intensly private. But the fact that you love him, and will, no matter what, is most important!
As several have said, his friends coming over is significant. We used to host Jessica's parties-for-the-masses and had 3 hard and fast rules. No drinking, no drugs, no sex. (We are a non-drinking household, so the rules still hold, and the kids appreciate it because the question is answered without any internal struggle. (As Jessica said last year about her cast party, I don't want to have to deal with X getting drunk, so the party is here.) We always had massive quantities of pasta around, but make-your-own-pizza works well, too. Make the crust ahead of time and set the kids loose with the toppings.
I almost envy you. It's a wonderful adventure. (But then, I have another boy to go through.)
CamiSu