When guys break up with you they often call or write to see how you “are”. It’s to relieve his guilt and to look like the good guy in all this. Also, the ego boost of believing that they are a heart breaker.

It’s your choice.

I would write “Great! I met someone.”

(“Someone” could be the postman, he doesn’t deserve full disclosure.)

It wouldn’t be the answer he was expecting, and he’d feel momentary shock at being so quickly replaced. But the trick is not to answer him back when he writes back a lame, “Oh, that’s great!” Leave him hanging. Leave him wondering. And don’t see or run into him. When you do enough time should pass (a year) that he sees you as a new independent person, not the ex girlfriend he dumped.