This Side of Heaven by Karen Kingsbury

Annie Warren always wanted the best for her son, Josh. But years of failure and bad choices created a heartbreaking distance that has grown far worse since the day Josh was hit by a drunk driver. Now on medical disability, Josh has put his life on hold for years, waiting for the insurance company to send a settlement that never seems to come. Worse, he believes the story of a scheming woman who claims they have a seven-year-old daughter named Savannah.
Despite the unlikelihood and complete lack of evidence, Josh dreams of being a father and is determined to one day claim the child. His family doesn't know the full story. They don't know what happened the night of the accident that was worth the chronic pain Josh suffers every waking minute, or that he is turning his life around. They haven't seen that Savannah's eyes are his, and they don't know how desperately the little girl needs her family.
When the settlement that rightly belongs to Josh is threatened, Annie sets out to defend her son. But she might find a treasure more valuable than money, one she never expected, one that is the greatest gift her son could ever give her--THIS SIDE OF HEAVEN.