There's nothing to figure out. So don't try. Next time he calls, don't pick up the phone. This guy is a wuss. Spineless. A human strawberry Danish. A wishy-washy prince who can't hold his white wine.
Some people aren't worth figuring out. They think they're jigsaw puzzles of wolves howling at the moon, but really, they're just the alphabet in wooden block form.
Before picking the phone up the next time he calls you, ask yourself the following question: is there something else I could be doing with my time rather than listen to this guy, like clipping my toenails? Or editing your "personal information" on Facebook? Or staring at the ceiling?
The answer is "yes."
He could be calling you, drunk, looking for some booty. Any man whose only booty call option is his most recent ex is lazy, creepy, probably both, and very soon, in retrospect, you will realize that you dated a lazy creep. These things happen.
He could be calling you because he's had a change of heart. If this was the case, he'd be telling you the same things sober. Don't trust people who turn into Dr. Jekyll and Mr. SoggyFace when they drink.
Here's why I think he's calling you though, and the clue is in your question. You were sad that he dumped you. But you were okay with it. See, you live in the future, and he lives in yesterday. You were and are okay with it. You've moved on. Stay in the future. It's nice. There are jetpacks.
This guy is regretting breaking up with you. And honestly, that's his tough sh!t. Maybe next time he'll be ready for a relationship, or learn that sometimes you don't feel ready, but you act like you are anyway and then you see what happens next. Stop talking to your ex and teach him a valuable lesson: you live with the consequences of your actions
Maybe in six months, after he's had some time to braise in the juices of his decision, you and he will have a polite cup of coffee and there will be a spark. But that's too far into the future.
For now, I have a better idea: when he calls you, don't pick up because you'll be out... on a date, with the girls, or on a date.