Britax is suppose to be coming out with a new seat that is harnessed but later converts to a booster (should be out in the next month or two). At the current time though, all the britax seats are either harnessed or booster seats, not combination seats. Your child is much safer staying harnessed for at least another year (preferably until age 5-6), at which time, he could possibly be moved to a booster (provided he is mature enough to stay seated properly).
Here is a list of the different seats and their maximums (only including seats that have over 40 lb weight limit and 17"+ torso height which excludes many seats). When looking at seats, look closely at the torso height as this is an indicator of how long the seat might last the child. Usually the taller the harness, the longer the child will fit in the seat (unless they weigh over the stated weight limit) as seats are outgrown when the child's shoulders extend above the top harness slot, the child's ears extend above the seat's shell, or the child exceeds the weight limit of the seat).
Some of these seats are pretty expensive while some are not as expensive. For inexpensive seats, you may want to go with the evenflo triumph advance, graco nautilus or cosco apex. Try out the seats and see which one seems to fit your child and fit your car best.
Britax Regent - top harness height is 20 inches. Can hold children up to 80 pounds. Requires a top tether after 50 pounds. Very big and cannot be taken on an airplane
Graco Nautilus - top harness height of 18.5 inches. Can hold children up to 65 pounds. This seat is forward facing only and converts from a harnessed seat to a highback booster to a lowback booster.
Recaro Como/Signo - top harness height is around 19 inches (Signo is 18 inches, Como is 19 inches) and harnesses up to 70 pounds. This seat also rear faces so it can be used for babies up to school age.
Sunshine Kids Radian - top harness height of 18 inches. Can hold a child up to 65/80 pounds (depending on the model you buy). This seat also rear faces so it can be used for babies up to school age.
Cosco Apex 65 (also referenced as Safety First Apex, Safety First Biltmore and a few other names) - top harness height is at 17.5 inches and can harness up to 65 pounds. After the child outgrows the harness, the seat does convert to a booster. This seat does require a headrest behind the seat so it will not work in cars without headrests or tall seatbacks.
Britax Marathon, Boulevard, Decathlon - top harness height is 17 inches. Can hold children up to 65 pounds. This seat also rear faces so it can be used for babies up to school age.
Fisher Price Safe Voyage (based on the Britax Marathon shell)- top harness height is 17 inches. Can hold children up to 55 pounds. This seat also rear faces so it can be used for babies up to school age. This seat is being discontinued so it is hard to find.
Evenflo Triumph Advance (don't confuse with the original triumph which is actually a very short harnessed seat)- top harness height of 17.5 inches. Can hold a child up to 50 pounds. This seat also rear faces so it can be used for babies up to school age.
Safeguard Child Seat harnesses to 65 pounds. Top harness slot is 19" high.
SafeGuard Go Child Restraint harnesses to 60 pounds. It's harness is a little different and it depends on the seat back. Requires a top tether for installation of seat.