Most Helpful Guy
I was never asked out by her when I was a teenager (I asked her out), but I would have liked to hear that she enjoyed talking with me, found me attractive, and would I like to do X, Y, and Z this weekend. You don't have to tell him he's attractive, but if it's a guy who doesn't have the greatest self-esteem, he'd like to hear something like that (assuming it's true). If he knows he's attractive, don't tell him he is.
It doesn't matter when you ask him out. If you do it before Valentine's Day you'll get to see what he does for you on that day, which also tells you a little bit about him.