But in fact he's a liar. The reason some will find his arguments "compelling" is because once Dershowitz has established the point he wants to make, he proceeds to manufacture a source to support it. That way he can never lose a debate, especially when he's not debating flesh and blood adversaries but a misrepresentation of their ideas, plus one or two straw men.
To be fair, Dershowitz doesn't make up his sources from scratch. He'll take a portion of an actual quote, leaving out crucial but inconvenient information, and will append to it an unrelated bit of argumentation, maybe from another quote that also exists, to obtain something which, while vaguely based on the truth, is a downright falsity.
One example is found in his recent article in The Jerusalem Post, which focuses on the figure of Amin El Hussayni, Mufti of Jerusalem during the British Mandate. The Mufti spent the WWII years in Berlin, where he collaborated with the Nazis to an unclear extent. This is used by Zionists to establish that the Palestinians bear some responsibility for the Holocaust, although there's no conclusive evidence that Hussayni's collaboration enjoyed support from the Palestinian public, and, on the other hand, Palestinian Arabs fought, alongside with Palestinian Jews, against the Nazis in the British-enlisted Palestine Regiment.
Of course, Hussayni wasn't any Arab; he was an Arab leader. But Avraham Stern was a Jewish leader, and in 1941 he offered the Nazis to enter the war on Germany's side (see here for a facsimile of the proposal). The Palestinians can't be demonized for doing what the Jews also tried to do (although the Zionist defense for this is that Stern has been repudiated in Israel, this is yet another lie: streets in all major Israeli cities are named after this frustrated Nazi collaborator).
In that context, Dershowitz claims:
[The Mufti] personally stopped 4,000 children, accompanied by 500 adults, from leaving Europe and had them sent to Auschwitz and gassed[.]
There indeed exists a story involving 4,000 children, 500 adults and the Mufti, but it's not as Dershowitz tells it. As Raoul Hilberg recounts in The destruction of the European Jews, all 4,500 people (from Hungary, Bulgaria and Romania) did manage to emigrate to Palestine (obviously, undetected by the Germans), and the Mufti protested to the Nazis, asking them to take active measures to stop further emigration. His objective was clearly to prevent more Jews from arriving in Palestine, not to block them from leaving Europe.
But it is the second part of Dershowitz's claim that is simply ludicrous. According to him, the Mufti "had them sent to Auschwitz"! Do you get it? The Nazis didn't actually mean to gas those Jews, but the Mufti, more Catholic than the Pope, arranged their transfer to a death camp! The sole idea that a guest of the Nazis could decide whom to send to the gas chambers, or that the Nazis would have needed any encouragement to kill the Jewish people they came across, would cause a roar of laughter among any audience even superficially acquainted with the Holocaust.
To summarize: the Mufti stopped 4,500 Jews who weren't in Europe from leaving Europe; and sent them to their death in Auschwitz against the Nazi reluctance to transfer Jews there. The case for expelling the Arabs from Israel is indeed overwhelming.