"- that someone so young is willing to sacrifice so much."
I don't think he looked upon joining up as you think.
From 1918 (and, from my father's history, well into the late 30's) a great deal of people would join up just for some form of clothing, regular meals and a comfy bed (most nights). Being paid was a real bonus.
As a hungry youngster Sir Norman Wisdom joined up in 1929. (from Wiki: "He first enlisted into the King's Own Royal Regiment (Lancaster), but his mother got him "unenlisted" as he was under age." After being thrown out of his 'home' he re-joined the army again in 1930.
That fact that this poor lad died of Flu is probably just indicative of his likely malnourishment and a susceptabilty for any illness going.
RIP kid.