M.W.
I just spent days and days researching this issue, looking for new car seats for my 3 year old & 5 month old. The following link was a huge help to me:
The top harness of your son's car seat should be at or slightly above his shoulders, so it sounds like he's definitely outgrown his current seat. You want him in a five point harness for as long as possible, so choose a seat that can accomodate that; many seats will allow 65-80 pounds in a harness. Most kids will outgrow a seat by height before weight, so you also want to pay attention to the height of the harness slots.
For our three year old, we looked at the Britax Frontier, Grace Nautilus and Sunshine Kids Radian seats before deciding on the SafeGuard Child Seat. http://elitecarseats.com/SafeGuard-Child-Seat.pro
Seriously, this issue consumed me for DAYS. Child safety seat technicians say that the best seat is that one that fits your child, your car, and is used correctly every time. So check 'em out & do some research & good luck making a decision! :)