The Warrior Strain.

Lonely young Lord Billy, who misses his dear departed mother, has nothing to do but lounge about the castle all day while his father, the Earl, attends to important affairs of state – until the noble lad joins the Army Cadets. Rising through the ranks to become the Lance Corporal of his squad, patrolling undefended points along the English coast, he's soon on the trail of German marauders.