The detail that best supports the inference that Will’s father wants to protect his son by keeping the mission a secret is:
“His father nodded. ‘Meanwhile, you must get those horses delivered. They’ll be used in the Continental Army.’”
This line indicates that while Will's father acknowledges the importance of the mission, he is also conveying a sense of duty and responsibility that emphasizes the stakes involved, suggesting a desire to shield Will from the full implications of his task.