We have the same problem and just bought a new car seat. The harness can be used up to 65 lbs. I don't remember what kind it is but it was on sale last month at Babies R Us for about $130 or $140. I have noticed though that his head is in an uncomfortable possition even with the harness, so I was going to get him on of those children's travel pillows (the ones that look like animals) to make it a little more comfy. His head seems to "flop" forward and to the side letting the strap rub on his neck. We didn't have this problem with the old seat because it had that "bar" that came down in the front and he would rest his head on that.
Just out of curiosity - do you have problems buying clothes? I have had the issue of wanting to get certain things (like the onesies and one piece fottie PJs made of lighter material) but they only make them so big. Since my "little" guy is in 4T and 5T at age 2 1/2, this has been a problem.