Christmas Guardian by Delores Fossen

Despite his dark past and dangerous edge, Jordan Taylor fell for the helpless infant left on his Texas doorstep with a frantic plea for help. The millionaire security agent had built a fortress around baby Gus...or so he thought until Kinley Ford showed up at Christmas fourteen months later.

Kinley wanted only to see her son; instead she'd led the killers on her heels straight to Gus. Killers who wouldn't hesitate to use him as the ultimate pawn. Kinley got to Jordan's heart, and after a lifetime of loss, he vowed to sacrifice everything to protect his surrogate son and the woman who'd ensnared him. Everything. Even his life.