Most Helpful Guy
Well you don't really give us much back-history to go off of, but I suspect "it's" been going on for longer than you expected. In other words, I wouldn't consider the date of when you broke up as the same as the date for when he was "allowed" to start getting over you.
He's been "getting over you" for some time now, even though it may seem so sudden.
Most Helpful Girl
I guess it depends on the situation beforehand. I left an abusive relationship after 10 years, and started dating again 2 months after I had moved out on my own. I didn't need time to grieve my relationship because, in my mind, I had been over it for well over two years. I'm not saying that's what happened with you two, but if he'd been struggling with the idea of leaving for a while, that could explain why he's moving on already. In his mind it's been over for longer than when he actually broke the news to you.