Your best options are the Graco Nautilus, Britax Frontier, and Britax Regent.
The first two seats are combination seats--they harness to a high weight and height and can be used as a booster w/o the harness. The Regent is a high weight harnessed seat, and it has the tallest harness slots of any seat on the market right now.
I would probably choose one of these, however, if he weighs less than 35 lbs and you want to keep him rearfacing AND you have a younger baby to hand the seat down to, the First Years/Compass True Fit reafaces to 35 lbs and has a really tall shell and top slots (comparable to those combo seats I mentioned).
Our ds is 3.5 yrs, 38" tall, and 30 lbs. He's about to outgrow the Britax Decathlon and Evenflo Triumph Advance (new one that is much taller than the old one) rearfacing, but has a lot of room for forward facing in these seats. When we need to hand down his seats to his baby brother, we'll probably get him a Graco Nautilus.
hths
henrietta