The climax of Shades of Gray occurs when Will is offered a choice: he can return to his original home with Doc Martin or he can stay with his Uncle Jed and the rest of his family in Virginia.
At the beginning of the book, this would have been a simple choice for Will. He's been orphaned by the Civil War and he's angry. He misses his family. He resents his Uncle Jed for choosing not to fight. By the time he arrives in Virginia to stay with his aunt and uncle, Will is filled with turmoil. He dislikes Jed from the moment he sees him.
Will builds a new life in Virginia over time and comes to understand Jed's perspective on war. He grows to enjoy the farm work and feels proud of himself for contributing. As his perspective on the war, the issues that caused it, and the people who fought in it changes, Will changes too. He begins to care for the family he initially resented.
When Doc Martin offers to take Will home so that he can live with him in the same place he lived with his parents, Will decides that he won't go. He'll stay with Jed and his family; he'll keep working on the farm. He's found a new life and purpose there.