A lot of people are saying it is Jacob reborn because MIB actually killed him (when he kicked him into the fire) and not Ben (when he stabbed him). I seriously doubt MIB spent hundreds of years trying to find a loophole and then would stuff it up by being the one to kill Jacob. Also the boy says that "you can't kill him". If it was Jacob wouldn't he say "You can't kill me"?? I feel like it might be Aaron because thats the only other character with blonde hair and blue eyes who is rather young and the whole argument that he is still younger then that kid doesn't really need to make sense since time and aging has never actually followed a set path on LOST. I also think its just as plausible that the kid is someone we haven't seen yet. Maybe someone in MIB's past.