Step 1: Understanding Dr. Sadao's father's attitude.
Dr. Sadao's father was extremely serious about his son's education and wanted him to achieve greatness. He placed great value on formal education and was determined to see Dr. Sadao succeed.
Step 2: Evidence from the story.
Dr. Sadao's father demonstrated his seriousness by ensuring that his son received the best education. He even sent Dr. Sadao to study abroad in America, where he could gain advanced medical knowledge. His dedication to his son's future was evident in the sacrifices he made and the clear expectations he had for his son's success.
Step 3: Conclusion.
Thus, Dr. Sadao's father's seriousness about his son's education was reflected in his actions, including sending him abroad to study, and his belief that education was the key to his son's success.
Final Answer: \[ \boxed{\text{Dr. Sadao's father was serious about his son's study as he sent him to America for advanced education, showing his belief in the importance of education for his son's success.}} \]