There is a constitutional impeachment process, for this office only. You need 2/3rd of the Bundestag to vote to impeach, and then the Constitutional Court to find him guilty in order to remove him. The Court may also issue an injunction keeping him from carrying out the duties of the office between the vote and the trial. However, he would have to be found guilty of violating the constitution, or committing a crime while in office, with intent in order to be removed.
Quite apart from the obviously absent political will to go through with this never-used procedure, I don't think the conditions are actually met. (Except that he signed the new election law without a hitch... hmmm... that probably was violating the Constitution with intent.
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