Call my ex-boyfriend, and I'll text him back and ask him if he misses me. He said yes. What does that mean?

It means that the other party is just perfunctory.

First of all, you and he are both ex-boyfriends, which means you two have broken up. Although the two people after breaking up can't be enemies, they can't be too friendly friends, because they both hurt each other's feelings, so they still dislike you.

In addition, you have noticed that every time you send a message, the other party doesn't reply, because he doesn't want to have any contact with you at all, and your sending a message like this will also affect his normal life.

Secondly, when you ask him if he misses you, he says that missing you is obviously perfunctory. If he misses you, he will call you, send you a message and come to see you. There is no need to wait so long.