I would get him his own seat.
I like the Recaro Signo.. I have one in Pink for my daughter and we just bought a blue one for my son.. This will be the only seat I need to buy for my kids, it goes up to 70 lbs.. My daughter is 2 years old and is only 25 lbs, so she has a LONG time before she'll out grow it..
Recaro is the best car seat maker in the Industry.. They've been around for over 100 years but they've only been making Convertible car seats for a few years. So not to many people are aware of Recaro unless your into modifying cars, etc.. The Recaro is a big seat, but it fits my 2000 Honda Civic Coupe just fine. What's also good about this seat is that Adriana has not figured out how to get out of it.. It's not easy to do.
Here's a few videos you should check out.
This is a review of the seat
YouTube - RECARO Signo Carseat Review
This is a Recaro employee talking about the Recaro company at a auto show.. This will show you the types of seats Recaro makes..
YouTube - 2007 SEMA Show - Recaro Booth
Oh, I forgot, Recaro makes car seats for Nascar.. If they make a seat for Nascar drivers, they know a thing or two about safety..