If your son is on the higher end of the chart for height/weight, I would get a convertible seat that has a high weight limit on it.
We have 2 Britax seats: The Roundabout and the Marathon.
My 3 year old son has outgrown his Roundabout so we exclusively use the Marathon now. I LOVE his seat. The covers are removable and washable. That is a plus for my son. He is currently 50 pounds (at 3) and he is NOT fat. He is super tall. He is actually 97% on the charts for weight and off of the charts for height.
If you get another seat as someone else mentioned...check the weight limits on it. It makes no sense to buy a $100 seat now and then spend another $200 on a seat when he outgrows the first one. Why spend $300 when you can spend $200.
In our case, we wanted 2 car seats to make it easier on ourselves. So we intended to spend the money on 2 but if I could go back and do it again, I would make sure that I got 2 Marathons rather than one of each. Don't get me wrong, I LOVED our Roundabout too but we can't use it anymore.
Someone mentioned that they found a closeout sale at their local Target for a Marathon. I'm not sure if they were being truthful, as the price seemed too good to be true but you could look if you happen to be close to one.
Also, some of the baby stores have sales...especially on seats that have discontinued covers. I can't remember the sites I was looking at but if I remember I'll post it.
Edited to add:
Here is a site with some specials on certain styles.
http://www.specialtybaby.com/britax.html
The site is actually
www.albeebaby.com if you want to look at other brands.
The cheapest I found for the Marathon was $216 with free shipping but it was the zebra style.