Dumbledore does find out Harry was a horcrux, but not until book 6. He did know some part of voldemort lived inside harry, he just hadn’t figured out what and how.
But the main reason he was preparing Harry was because of a prophecy made by Professor Trelawny which stated it had to be harry to eventually end Voldemort. It also stated that either harry will end voldemort or voldemort will end Harry, "for neither can live while the other survives."
It is quite fascinating how the story unfolds over the seven books. I would strongly recommend reading the books at least once if time permits. They are quite easy reads, written in very simple words.
I must start by buying the books... 😁😁😁
I would really like to have them