I like www.carsafety.org for car seat questions. here's the answer to your question from their FAQ page: ( by the way, I used a Cosco model for my son when he was that age - much the cheapest of the choices and ranked high by Consumer Reports at the time. I bought it at Target)
Q8: My child is over 40 pounds or too tall for his carseat, but isn't mature enough to sit in a seatbelt with a booster. Are there any options?
There are a few Forward-Facing and Specialty Models for this situation. First are the Britax Wizard, Boulevard, Marathon and Decathlon convertibles with 65 pound front-facing limits. There is also the Britax Husky, with an 80 pound harness limit, though a top-tether is required for above 50 pounds. Next is the Safety Baby Airway, a combination booster with a 50 or 55 (new version) pound harness limit. New models like the Cosco Apex also have a 65 pound harness. There are also some discontinued models that can still be found online or at ebay. They include the Britax Super Elite (up to 80 pounds in a harness), the Fisher Price Futura (up to 60 pounds in a harness) and the Britax Laptop (an approved restraint to 65 pounds). You may also use a Safe Traffic Systems RideSafer Vest or E-Z-ON Kid-Y harness/booster.