Seat belts on kids in boosters...

Ashley • Mommy of an angel, step mommy to two monsters. Baby #2 due March 2016. 😍 All four have my whole heart.
How old is your booster child and how do they wear their seat belt? 
My oldest step son is 4, he's in a booster, just the Normal bottom seat booster, I make him wear his seat belt correctly, in front of his chest. However, a LOT of people, including his mother, put it behind his back or their child's back? I feel like that way is not any better than not wearing a seat belt at all. Opinions?