Oh goodness! There are several seats that harness past 40 pounds.
For $110 there's the Apex 65. It has 17.5" top slots, a 65 pound internal harness weight and 100 pounds as a booster.
http://www.albeebaby.com/sa1stap65boc1.html
For $150 there's the Graco Nautilus. 19" top slots, 65 pound harness limit, 100 pounds as a high back and then a low back booster.
Can your budget go beyond that? There are others, though honestly the Nautilus would be my favorite, even among those priced more.
You can also apply to the Kyle Miller Foundation. They help families in need like yourself. Kyle Miller was killed in a car collision, and the family has started the foundation to help keep children in five point harnesses longer.
http://www.kyledavidmiller.org/pages/3007/Apply_for_Help.htm
I'd point you toward your local health department, but they'd probably put him in a booster, since I doubt they have anything that harnesses past 40 pounds.
Wendy