How do I talk to him? Long but I'm desperate!

I met this guy seven months ago. I'd never really had experience with guys, as far as feelings go. He kind of came out of nowhere, like he just popped in my life without invitation. I'm okay with it though, in certain aspects. Long story short, I fell in love with him. I was always afraid to say it because I wasn't sure if it was genuine or not, but I know for sure. I'd do anything for him. Hard part is, he has a girlfriend, yet is always talking to me as if we're in a relationship. He's told my best friend that he has feelings for me, as well. He will talk to me nonstop and then he'll cut it off. But then not even a week later I'll get a text from him, saying he misses me. I don't how much more I can take. I truly love him and don't want him to leave permanently, but I also don't think the way he's treating me is right. How do I talk to him about? I'm terrified.