Firstly, do you need a booster particularly? Is he ready for a booster full-time or would a high-weight harness seat be more suitable for him? I had kids who were ready to be in a booster around age 4, but I hear that many kids are not developmentally ready to sit correctly for a whole car-ride without a 5-pt harness.
I like the Graco Turbo-booster (high back) because it's really easy to adjust. We can switch the same one between 3 kids of v. different sizes really easily by adjusting the headrest, and it's easy to get threaded correctly. Even my 10yo and 8yo are now able to do it themselves. But, we bought them because we needed something super-narrow to fit 3-across in our sedan with a convertible. Not sure how well your kid will fit in it, as mine started to fill the seat width-wise at around 60lbs - they still fit, but not much room to grow if they get wider! (But, not an issue for us, because the older can fit in our van without in some positions, and the middle kid will, soon!)
Hope someone else can point you in the direction of other boosters that might be suitable, but I'd seriously consider taking him to try one out.