The RideSafer Travel Vest might be a little more appropriate, since it's for 30-60 lbs and ages 3 and up. That's a bit closer to your child's age. Plus, it sort of mimics the 5 point harness that he will already be used to.

Also, you can stuff it into an adult backpack that carries all of your stuff, too, along with any other necessities: snacks, diaper kit (if needed), drinks, etc. With the Boostapack you'll have an extra bag to lug, in addition to the one you'll probably be carrying.

I believe in the case of a vehicle that just has lap belts you should not use the Boostapack or the RideSafer. Your best bet, I'm thinking, would be to secure him with the lap belt. Boosters are designed to raise a child up high enough, so that the shoulder strap of a seatbelt goes across the appropriate place on the torso. The RideSafer adjusts the shoulder strap down to the child's level. Car-seat.org can confirm this.