E.K.
What seat are you currently using? Many less expensive seats are outgrown at 40-45lbs. I urge you to do you research and try your best to keep him in a 5pt harness as long as you can. We have a 2003 Camry and I have my 3yr old in a Britax Marathon and it's harness limit is 65lbs (although I'm sure we'll outgrow by height first), I have my 5yr old in a Radian 65, which also goes to 65lbs, but has higher harness slots than the Marathon. In my husbands car we have another Marathon for the 3yr old, and we have a Apex 65 for the 5yr old. It harnesses to 65lbs then is a booster until 100lbs. It's a ok seat but I've had trouble installing it in my Camry.
Take a look at the Nautilus it has been getting great reviews and it is around $150.
HTH
E.