Quoting Chuck Bass:" That is so odd because my son is 42 inches and his shoulders are almost level with the top slots. Did they make different versions?"
do you have it pulled all the way up? My daughter is 43 inches and 43 lbs and hers isnt level yet, we also have a nautilus. When he does get higher than the strap slots(if theyre all the way up, and they can be even), I would use it as a high back. The difference is that the high back holds the seat belt strap in place so it doesnt cut into their neck, whereas the booster just lifts them up so that the seat belt is in the right place. I would use the nautilus as a high back when he outgrows the 5 point harness until he cant sit in it anymore comfortably (like if he gets too wide for the seat, or if the seat belt holder stops holding the seat belt in an appropriate position for him.