Translation:I do not know when he will come here.
You're right Sydney. In English, we often use the present tense, especially with verbs like TO COME, to imply a future action or event. In fact, it is more common to hear "I don't know when he's coming" than "... when he will come". The context makes it obvious that what you're talking about hasn't happened yet. Have noticed that this is also fairly common in German.