Ann,
Of course he was not a Petty Officer;) He was a Seaman, but at the same time he was an assistant to the ship Bosun/Bootsmann (unter-Officer and head of all the ship seamen). So, you can call him with a few different, but close to each other variants (I do not know exactly): Seaman or Senior/Leading Seaman. But I've forgot the main thing, he was serving in the Guard (Standart was a prestigious Guard's ship) and you should call him Seaman/Senior Seaman of the Naval Guard.
Nicola.
P.S. Differences between Seaman and Senior Seaman are very small, so you can call him with both these ranks and of course add the word "Guard".