So if he eats a peach is it cannibalism??

No because the peaches are just peaches that provide immortality. The peach that fell from heaven is different story, when he fell he was filled with celestial essence which filled the seed inside. Reread the part where it fell from heaven and you'll see what I mean. The seed transformed into an embryo, then it kept growing into a baby while it fell through the crack of space and time.