D.S.
Hi C.,
I would consider the Graco Nautilus or the Britax Regent for your son. I would probably lean more towards the Regent because he sounds big for his age, and he would get to stay in a 5-point harness longer in the Regent.
Someone else recommended the SK Radian 80, but that definitely would not be my first choice of car seat for your son, as far as ease of use and installation goes.
I am a local car seat technician, feel free to send me a private message if you are interested in having the installation of your son's seat checked out, I can give you some info about local fitting stations that you can make an appointment with to get his seat checked out. :-)