how to DTR?

I've been seeing this guy for the last month and a half. I really like him and we've had sex a couple of times, so we certainly moved things along really quickly in the bigger picture as relationships go. But he's so respectful whenever we have sex and he's always so flirty, but we also go on dates where nothing sexual happens and have a good time just the same. Neither of us have dated for a long time, since we both ended serious long-term relationships over a year before starting dating one another. I'm way out of practice. How do I ask him what we are? I want to be able to call him my boyfriend and everything that comes with that label… I just don't know how to ask without freaking him out too much since we're both getting serious with one another for the first time in a while.