It has a 5-point harness and is suitable up to 85lbs. It only seems to be available in the US at the moment though. See it here
DS1 still fits in his stage 2 seat (priorifix) for now but I would rather he could stay in the 5 point harness for as long as possible because it is much safer than the seatbelt with booster seat option. Why are we always so behind things in the UK? :(