Why does this guy I'm dating call and text me when he wants but when I try to start conversation he doesn't really go for it?

Keep trying. Could be timing. Maybe he can't talk when you want to.

I'd also make him wait a bit the next time he texts. Always good to not always be so available. If texting him becomes too draining then stop dating him. That's why it's called dating. When you find a frog you move on.