A convertible car seat means that it can be used both rear-facing and forward-facing. Since your son is not yet one year AND 20 pounds, you'll need to have a convertible seat so you can keep him rear-facing longer (AAP now recommends keeping them rear-facing as long as possible).
Britax seats typically have the highest ratings and seem to the be the most popular, though they are pretty expensive. If you are looking for one that will last a long time, look into either the Britax Marathon or the Britax Boulevard. Both seats have a maximum weight of 65 lbs, so they will last a long time. My son is 3 and he's a big kid (tall and 40 lbs) and he still has plenty of room in his Marathon.
Convertible seats do not come with bases and don't just snap in and out the way an infant seat does. You either have to buy two seats (which is what I recommend) or strap it in and out every single day, which is not as easy to do with a convertible as it is with an infant seat.
If you want other brands to consider, people like the Sunshine Radian and the Recaro brands. I stick to Britax though.
K.
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