I believe he was the re incarnation of the husband

If anyone is familiar with souls and reincarnation, we are not supposed to remember our past lives because doing so would make this incarnation unbearable and impossible to endure, so we forget everything and are guided by karma and circumstance, however this sometimes fails, that is why some people can see the other side, something is broken in this system and that allows them to see dead people and be clairvoyant or psychic if you will. I feel like the child had a temporary failure of this system or had the memory triggered by the letters and then at the end of the movie he suppressed the memories since everyone told him it wasn't true and he doubted himself I guess.

And that's where we come to the problem of this movie, there is no true point of view, there is nothing in it, aside from the initial shot of the man dying and the boy being born, implying one is the reincarnation of the other, to make us believe or decide that he was in fact her husband reincarnated. They actually go through a great deal to emphasize that it was a lie because of the letters, when the director clearly intended otherwise.