There are two main types of booster seats: high- back and no-back (backless). I did a lot of research before buying and couldn't find any (objective) data indicating that one type was safer than the other. They both rely on the vehicle's seat belt system to hold the child in place during a crash.
A Parent's Guide to Buying and Using Booster Seats
I have one of each and use both regularly with my five-year-old son. The backless one is usually in the FJ. Since he's old enough to get in and out by himself, the booster is usually on the left side, behind the driver. But the FJ has a full shoulder-and-lap seatbelt in the middle position, so your son could continue sitting there if he prefers.
The backless models are less expensive and easy to use with the vehicle's seat belt. After having used a sturdy convertible baby seat with a five-point harness when my son was younger, I assumed that a high-back booster seat would be safer or somehow better. But after buying one, I realized that the back portion of the booster seat was fairly flimsy and wouldn't add much value in a crash.
It's really the vehicle's seat belt that holding him in place. The booster seat's job is mainly to raise up a child so that the seat belt crosses his/her relatively strong shoulder bones and not over his neck.
These are the two seats we have:
High-back:
Graco: TurboBooster® SafeSeat™ (Step 3) 8497MOS
Backless:
Graco: Backless TurboBooster® 8682SMT