Should I answer my ex?

An answer can reopen a conversation, but you decide how much of it you want to open. Practical unfinished business, curiosity and a wish to reconnect are different reasons; each can lead to a different kind of reply.

The old pattern and any stated contact boundary matter more than a rule about answering all exes. If a short exchange is what you want, a question about his purpose keeps it short without pretending that you already know it.