Since you both are talking so openly and he's asking you all kinds of questions, why can't you feel free to ask him some questions of your own!
There are so many possible explanations for his behavior, and that's why only he can tell us which theory is correct.
For example, it's possible that he's a player and doesn't want only one girlfriend. Maybe he's a guy who's afraid to get too close to any one girl, so he plays the field which enables him to dilute his attachments.
It's also possible that he isn't happy with his girlfriend and he's talking with you in order to figure out whether you might be interested in him, in which case he would then break up with his current girlfriend.
As you can see, the only sure way to know what's in his mind is for you to ask him. To do this, you might say, 'I know you have a girlfriend. I also notice that we've been spending a lot of time texting each other. I really enjoy your company and like how sweet you are with me, too.' I'm not sure if you aren't happy with your girlfriend and looking for another girlfriend or whether you're the kind of guy who would like to have two girlfriends!'
Then sit back and see what he says.