I have the same problem, tall & skinny kids. I put my now 6 y.o. (when he had just turned 5) into a Safeguard Go 5 point harness booster. I looked only at 5 point harness boosters and at the time the only ones available were that and the Britax frontier. The reason I chose a 5 point harness booster is because if you can google crash tests and compare the difference between them and a regular booster you will know why I chose the way I did. It should go without saying that the 5 point harness booster is so much safer for children than a seat belt booster - especially at that age.
I chose the Safeguard Go because it fit in the third row of my GMC Acadia better than the Britax. It has a smaller base width so it takes up less seat space and in a third row that is important. It comes with a travel case too and the seat folds up nicely and fits in the bag for travel. It will convert to a backless regular seatbelt booster when your child is age/weight ready for that step, so this should be the last safety seat he ever needs. It does require a top tether when using the back and 5 point harness and it has the option of using LATCH or threading a seatbelt through the back to secure it to the seat, much like a Britax convertible.
I did buy a Britax frontier eventually as an extra seat and also for use in my husbands Durango. It fits in his SUV better but I had an issue with threading the seatbelt through the back to secure it to the seat... if you follow the installation instructions the vehicle seatbelt is just not long enough to follow the path they show in the instructions. So we just use LATCH with it.
These are some of the places I looked when choosing my 5 point harness booster. I recommend you go to a store that has a good selection of them and try them out in your vehicle. The safest rated carseat may not be the safest in YOUR vehicle if it cannot be installed correctly because of seatbelts, LATCH capability, seat depth, etc. Also let your child sit in it and make sure she fits in it and is comfortable in it.
By the way, while looking up the information I used, I noticed the following statement in the first link listed below (from Sept 2009):
'In the latest ease of use ratings published by NHTSA, the SafeGuard Go received a four star rating, the highest of any
restraint in the category of combination seats used in forward facing mode.'
I hope all this helps.
http://www.safeguardseat.com/media/documents/news/safeGua...
http://www.youtube.com/watch?v=azgBhZfcqaQ&feature=re...
http://www.kyledavidmiller.org/our-recommended-seats.html
http://www.iihs.org/news/rss/pr122209.html