Your son's growth should slow down quite a bit soon. My first son was almost 20 lbs. at 6 months, about 22.5 lbs. at 1 year, and is just 26 lbs. now at 2 years old.
I would go with a convertible for him, to get the most from your money and also so you don't have to worry about buying a new seat for a few years.
The Britax Marathon/Decathlon/Boulevard ($220-$300) and the SunShine Kids Radian ($180) are great choices, they go to 33 lbs. rear-facing and 65 lbs. forward-facing, they have tall backs and high top slots, and they have a versa-tether that can be used rear-facing (Britax's and the Radian are the only seats that have this). Britax's are very easy to install and use and are a favorite around here. The Radian is a great seat IF you can get a good install...it can be difficult, if not impossible, to install in some vehicles...it's definitely a seat you would want to try before buying. All of these would last until around 5 years old.
The Fisher Price Safe Voyage Deluxe ($150) is made by Britax, but under the Fisher Price name. It goes to 33 lbs. rear-facing and 55 lbs. forward-facing. I believe it's made on basically the same shell as the Marathon, so it's the same size and has the same height top slots. It does NOT have built-in lockoffs or a rear-facing tether though. This one would also last until about 5 years old.
If you are looking in the $100 range, there is a new seat called the Safety 1st Uptown ($99)...we don't know a whole lot about it though. It goes to 35 lbs. rear-facing and 40 lbs. forward-facing. It seems to be similar to the Cosco Scenera, but with more padding/nicer cover. I'm not sure, but I would say this one would last until 3-4 years old, depending on the height of the back and the top slots.
The Cosco Scenera ($40) is a great value, BUT has next to no padding. It has a tall back and a 35 lb. RF weight limit, so it's great for extended rear-facing, then it goes to 40 lbs. forward-facing. This one would last until about 4 years old.
I think I've covered the most often recommended convertibles here! If not, someone else will add what I've missed!
If you don't already know about extended rear-facing, the American Academy of Pediatrics recommends rear-facing for as long as possible (to the limits of a convertible seat) for the best protection, which would be to 30-35 lbs. OR when the head is 1-inch from the top of the carseat. My 2 year old is still happily rear-facing and will be until he outgrows his seats rear-facing and so will his little brother! A lot of us here have our little ones rear-facing well past the outdated, bare minimum of 1 year AND 20 lbs. Here are some links for you, if you would like to check out the information:
http://www.cpsafety.com/articles/stayrearfacing.aspx
http://www.thecarseatlady.com/car_seats/rear-facing_seats.html
http://www.childrestraintsafety.com/rear.html
http://www.windsorpeak.com/dc/dcboard.php?az=show_mesg&forum=13&topic_id=44503&mesg_id=44503&page=2
And a video that contains crash test footage of RF vs. FF
http://youtube.com/watch?v=kRP7ynNI8mI
I hope this helps and sorry it's so long!!