Should I call him for closure?

Yes you should call him

I would not give him the satisfaction, screw him, he doesn't pick up your calls and you want to call? You've only invested a month of your time, I would not invest anymore, I know you thought he was different etc etc but now you know he's not! Why let him know your hurting? So he can gloat? If he cared about how you felt he would be calling you up! Not to be mean, but what kind of closure do you need after a month? It sucks I know, but just move on forget about him and don't call, see if he ever calls you. Why do you want to know why he left you, will it matter? Will it make him get back with you? Sounds like to me, he had his fun with you and now he's moving on, I mean you only dated for a month and you were sleeping together already? Not to judge I mean hell we've all done it, but maybe that was all he was after in the first place? Like I said don't give him the satisfaction of a call, just move on, act like you could care less, I think you'll get more of a response this way anyhow, if that's what you looking for, calling him up and asking him direct will only get you a line of crap from him, since he already has acted immaturely, he will tell you whatever he can to get off the phone with you and come out looking like the nice guy, don't fall for it, don't call! Good Luck!

Call him but you said yourself he's not responding to your calls anyway. But for your own sake and to get the closure you need, try and call him see if he answers.

Now from my own experience, when my ex started acting exactly like this (moved me from his #1 on myspace, stopped calling me as much, etc) the reason was. Tada! He was cheating on me. So be prepared for him most likely having someone else in his life.