There really isn't one that you will be able to use the rest of his carseat life. Don't buy an alpha omega because they only harness to 40lbs and from what I have heard they are a horrible belt positioner booster seat. You can get a convertible carseat and rearface to 35lbs and there are some that harness to 65lbs like the britx marathon, the frontier and the first years true fit or if that's too much for you. You can even buy a cosco scenera they rearface from 5-35lbs and foward face from 22-40lbs, they run around 40$. Then get a graco naulitus, they are a foward facing 3 in 1, it harnesses to 65lbs, then turns into a belt positioner and then a backless booster and they run around 150-180$.