Most Helpful Girl
Well I highly doubt any of us strangers can give you a precise answer because None of us KNOW your ex boyfriend. HE could be doing it for a NUMBER of reasons.
If you two are still on talking terms, its safe to assume that you are still friends. Maybe he considers you his best friend or a close friend and wants to share the happy details of his life.
He's trying to make you jealous or hurt you to some degree.
He still thinks you have feelings for him, and without directly rejecting you, he's letting you know that he's moved on and is happy.
Without knowing him personally, its hard to explain what he is thinking and what his intentions are.