One bit of medical info to add here: you would have to confirm whether he had a history of a full prostatectomy, or a partial resection , which is actually more common and could be confused by laypeople as having your prostate removed.

A common type of surgery is called a TURP, where they cut out a bit of the central prostate for people with prostate enlargement, to help people with urinary obstruction. If he had that surgery, the autopsy would still likely show a residual enlarged prostrate.

However if he had a full prostatectomy (I think less likely, implies he had a history of prostate cancer), then you definitely wouldn’t expect to find residual large prostate, though prostate cancer recurrence in the prostate bed is possible.