I miss him. Should I write?

Missing him tells you something about your own feeling; it cannot tell you whether contact would be welcome or useful now. A message can express that feeling, ask for a conversation or reopen an old question. Those are different choices.

If no contact boundary is in place and you want a simple connection, a small honest line is enough. Keeping it as a draft is another option when you have not yet decided what you want from sending it.