Okinawa was assaulted on 1 Apr 45, and Ie Shima on 16 Apr 45. If he left the Army in October, 1945, I'm not sure how it would have been possible for him to have been "on Okinawa for 21 months".
If he saw Ernie Pyle get shot on Ie Shima, he may very well have been in, or attached to, the 77th Infantry Division. They assaulted and secured Ie Shima.
You said he, "was in three major battles in the Pacific". Do you know the other two? If so, by comparing units involved in the three campaigns, and by a process of elimination, we might be able to establish his outfit.
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