At 19 he doesn't have any legal right to live in your home.
Having been through something similar, I feel for you.
You have asked him to leave. If he does not, he will be trespassing. Even if he were paying rent, you could require him to leave. It's your house, not his.
It's then a matter for the courts. Unfortunately, this will cost you, although you'd probably get costs awarded.
It's a serious step though, and perhaps you wouldn't wish to go that far. Though if he calls your bluff, and you back down, you could be stuck with him for years.