“I won’t fight against Italy” Your grandfather would not have had to worried about fighting against Italy after September 1943. Mussolini was overthrown and the new government of Italy, under General Badoglio surrendered to the Allies on Sept 3, 1943 and joined the Allies against Germany on Oct 13, 1943.
Thus he could have been sent to Italy to fight WITH Italy against the Nazis and diehard Italian Fascists. Or he could have been sent to the Pacific Theater.
The armed forces tended to send Germanic-heritage soldiers to the Pacific. My gramps was on the Hancock, for example.