I.M.
You could try the Evenflo Titan, they sell it at Target, it's supposed to last up to 50 lbs, and got really good reviews from Baby Bargains (though if your son is on the tall end, he might outgrow it height-wise). Or, if you have a little more cash, Britax is doing their once-a-year sale, so you can get the Marathon for just over $200 at Amazon or Diapers.com (with free delivery).