well If he decides not to join I'm very sure he will have a good reason. like maybe so Lord Voldemort does not read his thoughts. Or for the protection of the Order. you never know who is trying to kill harry. like maybe if harry does vanquish the Dark lord maybe somebody will take his spot like Draco Malfoy or Lucious or some one. and they may try and get revenge on harry and the order.
i don't think he'll be able to officially join for the same reasons that phoenix stated, but you never know. so many things have changed now with everyone knowing voldemort is back. but i do think that dumbledore will let him in on most things, especially stuff concerning him (harry) after he's got his occlumency down. dd knows not too keep too much from harry now (i hope)
In that case, then they should enlist his help. He does have a track record in defeating the Dark Lord. I had assumed he would be defeated by then, as Book VII is going to be about Harry's final year, correct? So there won't be any more story for Harry to get Voldermort, if he fails to do so by that point in time.