The ex-boyfriend said "take care" to himself. How should he answer?

I can't figure out why you are still in contact with your ex-boyfriend. If you are not friends now, cut off contact as soon as possible. Don't lose everything.

If you and your ex-boyfriend are friends now (at least on your side), then I think you can reply like this: "Don't worry, he will take good care of me." Answer like this, even if your ex-boyfriend still likes you, he knows you have a boyfriend, and he will slowly give up on you. If he is your friend now, he will be happier if you answer him like this.