Why does a guy say he wants space and not a serious relationship?

Because that's men for ya. Go and have fun and talk to these other guys. The CHEEK of him to say he hears your talking to other fellas, too right you are! Your hardly going to wait until he decides what he wants. And say that to him I don't understand the term all people use "serious relationship" tell him you just want a relationship then! Lol Go with the flow, plenty of other fish in the sea especially at your age.

I think he's experiencing the withdrawal period. Almost every guy will draw away from their significant other at some point in their life. Read "Men are from Mars and Women are from Venus". That should help clear things up.

He's feeling jealous and possessive because you're talking with other guys. Not sure how to interpret this. It could be a good thing, but it could be a bad thing too.

Give him a few more days and see if things improve. Call him and set up a meeting somewhere so you could discuss about things between you guys.

Good luck! :)