John Kipling was not only very young (but not too young to serve), he was also even more myopic than his father, falling well short of British Army recruiting standards even in the Great War.

But "My Boy Jack" was actually written about John Travers Cornwell, known as Jack, who was a 16-year-old "Boy First Class" when he died at his post aboard the cruiser HMS Chester during the Battle of Jutland. Awarded the Victoria Cross posthumously, Boy Cornwell was the youngest VC of the Great War, and the third-youngest ever.

John Kipling was a Second Lieutenant in the Irish Guards when he disappeared in September 1915, during the Second Battle of Loos. I don't think his father would have used nautical symbolism in a poem about him. Also, to the best of my knowledge, he was not called Jack.