Soldier's Secret Child by Caridad Pineiro

At eighteen, Macy Ward had turned to Fisher Yates for a single night of comfort and then Fisher joined the military while she married another man. Years later, she is a single mother, and Fisher is back in town, unaware that the troubled teen with Macy is the result of that one passionate, unforgettable night.

Macy knows that she should tell Fisher. But the military is his life a life that never seemed to include the desire for family. Until her son's disappearance brings them together in a race to save him and forces the ultimate decision between duty and lost love.