Both my parents and my in-laws have versions of the Cosco High Back booster carseat. DS is 3.5 years old and getting close to 40 lbs, which is the limit to which they can be used with the 5 pt harness. I am not comfortable with him using them as boosters at his age even though he doesn't ride with them all that often. I was thinking of suggesting that they get Graco Nautilus car seats since they harnes to 65 lbs and can then be used as boosters after that. I'm hoping they won't have to buy new seats after this, but is the Nautilus a good booster? DS is average height and weight for his age. Would this be a good seat for him as a 5 pt harnessed car seat? Would it be a good seat for him as a booster when he is older? Would this most likely be the last seat they would need to buy for him? Are there any other better options for him? Thanks!