A rear facing car seat (infant carrier too) is outgrown when the child goes over the weight limit or there is less than 1 inch of hard shell above the childs head. Until either of these happens, the seat is not outgrown. Legs over the end of the infant seat is not a problem, and not a danger, and not an indication of the seat being outgrown. While rear facing the harness should come from below or just at the shoulders. It is best to remain rear facing as long as possible, but the bare minimums are 1yr and 20lbs.
Now, that being said, most infant seats (the lower weight ones, 22lbs) are outgrown around 5/6mos of age. What you want to purchase is a CONVERTIBLE car seat so he can remain rear facing as long as possible. The AAP (and NHTSA and SafeKids) recommends children remain rear facing in a convertible car seat until at least 2yrs and 30lbs, or until the child outgrows the seat rear facing. Convertible car seats on the market today will rear face until 35lbs, a few to 40lbs, and one to 45lbs. Most children can remain rear facing until their 3rd birthdays in the US. In Sweden, children ride rear facing until 50lbs, and they have had less children die in 10yrs in auto accidents than the US has daily. That speaks volumes!
There are many many car seats on the market that will rear face your child, then forward face, and last your baby until he needs a simple booster around 6yrs of age. A child should remain rear facing as long as possible, and then in a harness as long as possible, and in a booster seat until the child is 4'9" tall, or around age 8.
First Years TruFit, $150 will rear face to 35lbs and has a super high shell, and will then forward face to 65lbs. This seat will harness most kids to age 6/7yrs old. GREAT value and highly recommended, but may not fit with 3 across.
Graco MyRide, $150 will rear face to 40lbs and forward face to 65lbs. Will fit most kids to age 5/6 in the harness and is a GREAT seat for extended rear facing. Its super plush too, very comfy. I really like this seat.
Sunshine Kids Radian65 is a narrow seat and will easily allow you to fit 3 across. It will rear face to 45lbs and forward face to 65lbs, and fits most kids until age 6/7 in the harness. They run around $250.
Evenflo Triumph ADVANCE (make sure its the ADVANCE, not the regular triumph)- will rear face to 35lbs and forward face to 50lbs, fits most kids to age 5 in the harness. $150. Easy to adjust with no rethread harness, just a twist knob on the side.
Britax Marathon/Boulevard- Will rear face to 35lbs and forward face to 65lbs. Most kids can use this harness until 6yrs old. We had one of these andit was by far my favorite car seat. $280.
Those seats are what I would recommend. Steer clear of the '3-in-1' seats like the Cosco Alpha Omega Elite or the Eddie bauer 3 in 1. They are ok for rear facing, but they are generally outgrown forward facing before a child is mentally and physically ready for the booster portion, and the seats are so wide that they hold the seatbelt away from the child too far in booster mode and make very bad booster seats. They aren't worth the money, you are better off buying one listed above and using it to the limits and then purchasing a dedicated booster that will fit your child better when she is older.
So the seats I listed above are the ones I would recommend, you get the biggest bang for your buck, the seats are both comfy and easy to use, an both will last your son until he is at least 5/6yrs old in the harness. However any convertible car seat will work for him... but just beware of the 3 in 1 seats, you can get way more for your money with the other seats.
A few things for you to google while researching car seats- Extended Rear facing, internal decapitation, seatbelt syndrome. Go to the Safekids.com website, and visit www.carseat.org too. The best time to be informed about car seats is right now.
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