I'd recommend the First Years True Fit. You can usually get one at Target or BAbies R Us,or online at Amazon, etc.
It will last until 35lbs rearfacing (you should keep him RF as long as possible, it's much safer) and for a long time forward facing (until his shoulders are even with the top slots or until 65lbs). For most kids that gives you good odds of making it until booster age (6ish) without another seat. It runs about $180.
Important things to look for in a new seat: RF weight limit of 35lbs, tall shell, tall harness height (most kids outgrow seats by height before weight), high forward facing weight limit. And price of course.
Other good choices:
Sunshine Kids Radian (~200), Britax Marathon (~300) or Boulevard (~350), Evenflo Triumph Advance (the advance part is important) (~100)
Of those, the TrueFit or Radian will last the longest, Followed by the Marathon/Boulevard, then the EFTA.