A rear facing car seat is outgrown when the child reaches the weight limit or there is less than one inch of hard shell above his head. So he's probably outgrown it in height. My boys wre out of the infant seat at 4mos and 5mos of age because of height. :)
Here are some seats to get you started in your research. All the ones I list are extended rear facing and extended harnessing seats, meaning you will keep your child rear facing beyond 1yr and 20lbs. New AAP recommendations are 2yrs and 30lbs or until the rear facing limits of the seat are reached! Extended harnessing means keeping your child in the harness beyond 4/40lbs, safer for the child.
Britax Roundabout50 ($160 at Target, harness to 50lbs, rear face to 35lbs),
Sunshine Kids Radian ($200-$300 harness to 65lbs or 80lbs depending on model, and rear face to 40lbs or 45lbs depending on model)
Graco MyRide 65 ($150 harness to 65lbs, rear face to 40lbs)
Evenflo Triumph Advance ($130 harness to 50lbs, rear face to 35lbs)
Avoid the 3-in-1 seats (except the Symphony suggestd below, it is actually ok) because they are ok seats but are typically outgrown fast in height and they make terrible booster seats, failing every crash test. Namely the Cosco Alpha Omega.
Please google EXTENDED REAR FACING and JOELS JOURNEY for more information on keeping your baby safest.
Also, for rear facing the harness slot should come from just at or BELOW his shoulders, and the chest clip needs to be level with his armpits, and the harness should be 'snug as a hug'. NO BULKy clothing in the car seat either, anything that causes you to have to loosen the harness to accomidate is TOO BULKY for a car seat.