how did he not find out earlier?

Although I'm adopted, my three cousins never knew they were adopted. *I* knew that they were adopted from a young age, infants and toddlers. I was tasked never to tell them and I never did so. I figured they'd learn about it one day and they did. Unfortunately, they learned the secret at the worst of times. They were very angry that they hadn't been told the truth by their parents. It's not as though they would have been surprised; their folks were foster parents for many other children.

The oldest daughter had a car accident in which one of her own children was tragically killed. She went looking for her own birth certificate for some reason and when she found it, the information was there for her to see that she was adopted. It was as easy as that for her to uncover the family secret. She was beyond betrayed in her feelings.

It took several years for her to calm down enough to speak to her mother again. Don't keep family secrets. It's not worth it. Tell your children they're adopted; they'll find out eventually. I always knew. It's for the best.

My point is, how would he have known he was a clone? No one knows when they are adopted. There's no X marking the spot or an automatic birthmark or trademark telling you that you don't belong. Well, there was with me. I was the sole redhead in a group of people who were either blond with blue eyes or had dark hair. I was definitely a sport.