Do you know about how tall he is now?
The biggest issue with the newer Britax seats, is that the shells are a bit shorter than the older models (like the Classic Marathon still sold at Target), so they are "sometimes' outgrown earlier. It also depends on how long you plan on rear-facing. I plan on rear-facing my son to 3, and he probably won't make it to 3 in our new Britax seats. I think he has about 1" of growing room left, and he might reach that before summer. My son is about 35" tall, has a 14" torso and is 2.5.
On the other hand, my daughter is going to be 5 Wednesday, and still fits rear-facing in her Classic seats. She is 40" tall, but I am not sure on her torso.
Other seats that are good for taller kids are the Graco My Size 70/Size 4 Me 70/Headwise 70, Diono Radians and the Clek Foonf.