The following all harness past 40 lbs. It really depends how long you want him in a 5 point harness. The longest possible is the best, so I would go with the Britax Regent (we have that one). The Regent harnesses to 80 lbs. The other Britaxes are nice (Britax Boulevard, Marathon, Decathalon, etc) harness to 65 lbs, but most often kids outgrow it by height before weight. Most kids outgrow those seats around 45-47 lbs. So, you get more bang for your buck with the Regent. Britax is having a sale very soon (next week or two) and it's on line only, so make your decision fast and you can save some money. Check out www.car-seat.org if you want to know exactly when it is and if you want more reviews on which seat to get.
Your other options are- that haven't already been mentioned:
Recaro Como or Signo- harness to 70 lbs
Graco Nautilus- brand new, harnesses to 65 lbs. then converts to a booster- around $150- in some Walmart's now, more later this month
Coming out soon, and looks awesome- Compass True Fit (look at amazon.com for pics)
As with the Radian 65 or Radian 80, I bought one and had to return it because the straps are closer together than other seats and ended up digging into my son's neck. Many people have complained about that, and given your son seems a tad bit on the bigger side for his age, I would probably forgo that one. It isn't around in any stores here, so you have to buy it on line and pay shipping to return which stinks.