EXPLAINING THE WHOLE MOVIE *spoils everything*
first, the kid was not sean, ever, not even for a moment, obviously, since sean was cheating on anna and would have gone straight to clara instead of anna, which makes it completely obvious right there, BUT, more proof... the kid saw clara bury the love letters sean gave to her (they actually show him walking behind clara after she buries a box which contains those letters around the ending of the movie, only the timeframe of that scene was actually right before the kid began believing he was sean [the beginning of the movie], not the night before). (these were the love letters that were written to sean by anna. sean never opened these letters and gave them to clara to prove he loved her more, as clara explains at the end of the movie. and yes, he did love her more, why the hell else would be hand over alllll the love letters written to him by anna completely unopened? not a very loving thing to do, is it?)
the kid then unearths them and reads and memorizes details written in those letters (this is why he couldnt just go in and recall certain conversations or talk about the lectures he used to give). no, he didnt read and memorized details because he wanted to find a way to trick anna, but he read them and fell in love with anna and her writing (much like anyone would have etched in their memory a favorite story and author when they were kids). and so he became sean through the love letters and fell inlove with anna. and then the movie unfolds as it did.
to explain better the part when clara gets to anna's place and the kid opens the door to her: the kid opens the door. clara is there. the kid is shocked. she shows him her hands are dirty. the kid realizes clara went to see if the love letters were still buried and is kinda paralized. thats why when clara asks him to show her where the bathroom is and then asks him to dry her hands, he does so without thought. he's afraid clara will tell anna about him digging up the letters. clara gives him her new address and the kid actually asks her to not tell anna (no, this was not about the cheating on anna with her. keep in mind, he didnt even know who clara was, which he admits around the end). they both know clara expects him to show up the following day with those letters, which is why he had them in his bag. another thing, how could he possibly be sean and not know the person (clara) he loved more?
but by the end of the movie, its pretty obvious that he's not sean, but a kid who took on a second persona. anna is just completely devistated that she's getting married and its not to the real sean (u can see how happy she was, right up until she saw joseph during the photos)... the end shows her having a breakdown of sorts at the beach (the first place sean and she ever met), tying it all together.
i just wish they showed the love letters and the parts he memorized and used, but this movie seems to be more of an open-ended piece, deliberately trying to cause conversation. i just think it would have been better if explained in the movie. she lost sean 10 yrs ago and the child is 10 yrs old... kinda makes u think it might be possible the kid really does have the reincarnation of sean's soul in him... but if it wasnt that way, the movie wouldnt be half as good.