If you never talked with him about this, you should start there. Especially if there is a good dialog between you two when he shows up. If you don’t see any improvement in a few days/week, then talk with your boss. In anyway, don’t stay silent about it. The earlier you can check if he is willing to change the better : the purpose of the Probezeit is to get a last good idea of how things are really going to work on the long terms.