He found out in the battle against Pain, when the 9-tailed fox took over his body so much, that Naruto was forced into his own subconscious. There, the 9-tailed fox almost tricked him into opening the seal completely, letting him 'destroy everything that hurts you', and 'save him from the pain'. The second before he tears away the seal, the fourth hokage shows up, and stops him from doing so, telling him that the seal is made so that if 8 tails appear on Naruto (8 tails is the last stage before the demon takes over his body), he will show up inside Narutos subconsciousness. Even though he states that he didn't want to see the 9-tails ever again, and says that it's evened up by him being able to see his son, Naruto doesn't understand that the son is him. Only when the fourth says his name w