He has to become smart and shaft the Merkelite from CDU. That way, he can welcome back original CDU electors from AFD.
That's the aforementioned political dead end of German politics: the CDU has lost so much trust with right-wing voters that they won't win them back from the AfD without actual, tangible bills and executive actions - but as long as they rule out coalitions with the AfD, they are destined to coalesce with left-wing parties where such tangible action is impossible. On the flip side, the AfD has also attracted a couple of voters who used to lean to the political left, so that left-wing governments against the CDU have also become unrealistic.
Hence, the only workable coalitions left are murky alliances across political camps in which gridlock reigns supreme. These inefficient makeshift coalitions in the middle strengthen the fringes, rinse and repeat. The past 3.5 years under a "traffic light" coalition between Social Democrats, Greens and the libertarian/classically liberal Free Democrats were an experiment at finding another way out of this impasse, but it was a disaster. Now, the Free Democrats are out of parliament and on the verge of political extinction, so this option is dead and buried for the forseeable future.