How do you know when I a guy is serious about you?

Ask him about it. He has either been burned, and wants to be sure you won't bolt on him, or he has a "circle" he doesn't trust to be accepting of you, so he is trying to protect you as long as possible, from them. There are an infinite number of other possibilities, but these two, or a combination of them seem to be most likely. You may want to pray about it beforehand and let God influence the wording, and the situation so he is open to it.

He said it point blank to you. Confused Diva? I think not. Let him be. Sure, you can be
friends, that is friends with no benefits. Your life is going forward and it doesn't include
players. When he decides to grow up, on the same introduction agenda as you, procede
as your heart directs you, just don't forget your mind. My dad always said that because
I tended to use my heart when it came to guys, that I should pretend I was a semi and
use business sense. In a way, he is right. By the way, that notion would work both ways.
Use this time to invent who you are, who you want to be. I think it to be great that you two
have found one another. Maybe, this concrete will help to reaffirm his bedding. Good Luck!

Pray that you find someone who isn't a wimp about commitment.