My son and I have just finished Halo 4, we played every level together coop on Heroic. We always played a full level before saving at the start of the next level to continue the next day. When we finally finished, I got the achievements for completing the game but he didn't. We both have all 8 achievements for the levels but somehow it doesn't equate that to completing the game for my son. He's not very happy, why hasn't it credited him with the completion cheevo?