If he had served on HMS Triumph in the Dardanelles at the age of 17, presumably he would have been a Boy Seaman before becoming a Midshipman. However his name does not appear in the records below:
UK, Royal Navy Registers of Seamen's Services, 1848-1939
His Temporary Midshipman details are on "Admiralty: Royal Naval Reserve: Officers' Service Records" dating from after the Triumph was torpedoed.
If he was in the Dardanelles, perhaps he was on a merchant ship and in the RN Reserve after leaving Worcester and witnessed the sinking of the Triumph.
My interest in this topic is due to my grandfather serving as an RN Stoker and surviving the sinking of both the Triumph and the Majestic.
Tony