This dude was never an atheist. End of story. There are way too many pathetic attempts by people who think that not attending church is being atheist. Real atheists don't convert to theism.
He was possibly an atheist by the strict definition of the word: He didn't believe in a/any god. However, I highly doubt that he ever gave the issue much thought and/or even to this day knows what the word 'atheist' even means.
At any rate: Your statement might, to avoid fallacy, be better rendered as : Atheists who have given serious thought to the subject and understand it
don't convert to theism.