Your ex boyfriend is a bit on the possessive side, isn't he? He's the guy that tells you that he wants you to be happy and in the next breath gets incredibly upset when you decide to move on and date someone new. You're exasperated, as you should be. You two aren't together anymore so he has no claim on you, right? That's the way you see it but chances are good he views it in a much different way. If you're tired of listening to your ex tell you that he's over you yet he doesn't want anyone else to have you, it's time to jump off that emotionally exhausting train. You have every right to date whoever you want and once you understand why your ex is still being so territorial you may decide that he's actually still the guy for you.
If he doesn't want anyone to have you he still has feelings for you. It's really that simple. As much as he may claim that he's happy that you two have broken up, there's obviously something else going on beneath the surface for him. Typically when a couple decide to call it quits, they go through a short mourning period before one of them jumps right back into the dating pool. The other soon follows and their relationship and the time they spent together becomes a distant memory. If one person can't let go completely they'll have enormous trouble knowing their ex is dating anyone. They don't care who that other person is, they just don't like the idea in general. That's exactly what your ex boyfriend is experiencing now.
You can absorb his behavior as romantic if you're inclined to. For any woman who has been struggling to let go of her ex emotionally, knowing that he's possessive is actually a plus. You should talk to him about this but expect him to deny feeling anything beyond concern for you. If he's unwilling to admit that he's wild about you still it may be time to cut all ties, simply because you'll continually be faced with his input about the men you date until he feels he can let go.
If you're ready to move on it's time to disengage yourself from your ex boyfriend's life. That means you need to stop sharing so much with him. Your relationship is over and if he continues to contact you to garner information about who you're dating or what you're doing, you need to step up and tell him that it's not his concern anymore and wish him well.
In the case of an ex boyfriend who is learning about your current boyfriend through mutual friends, you need to rethink the people you are closest too. Who you are interested in romantically and where that relationship leads is not your ex boyfriend's business. Cherish your new relationship and don't share more details than is necessary with anyone who contacts your ex. In time, your ex will bore of this and turn his attention to someone new.