The idea that it's none of your business that they're dating is bullshit. You and she have a history, and your 9and her) ability to handle that history while she's dating a friend of yours will set the tone for your continued friendship with the dude that's now dating her.
Now, that said, it's not required for him to seek your permission, nor is it your place to withold it. You and she broke up, fair play to him. But as a friend, if he actually values your friendship and feelings, it is his responsibility to tell you.
He should know that dating a friend's ex is opening a pandora's box of akwardness and drama, and should at least have the decency to tell you about it. It's as simple as sitting down with you and saying "I want you to know I'm dating Jennifer. I know you have a history, and this could be akward, but I like her, and I want to pursue this, but I still want to be your friend." and work from there. If you give him your blessing, awesome. If you have a problem with it, you sit on it, and try to work past it yourself.
The fact that he did it behind your back suggests to me that he didn't care about your feelings, and acted selfishly. He cut you out of the loop without even giving you a chance to stay in it. Fuck him.
Acting in a mature, adult manner. It's what's for dinner.
The bro code isn't "bros before hos" it's "treat your bros like bros, even when a ho is involved."
-m