No he was not. He was born with the ability to possess it per US statute, however, he did not acquire it until his parents moved back to the US.
He had all the rights and privileges of a US citizen from the time of his birth no matter where he resided.
Whether he processed the paper work or not is irrelevant to whether he is a US citizen at birth.
Certainly processing a CRBA makes it easier for one to prove one's citizenship. But it's NOT required and there are other ways to prove citizenship.