DS10's favorite puzzle when he was that age was a Martin Luther King Jr judy-puzzle, it only had 12 pieces but they were odd shapes and really fun to assemble. He called it "the king puzzle" and he really loved it lol I still have it of course!

I picked up a ton of those judy-style puzzles at thrift shops and he liked them alot. They are heavy duty masonite or MDF type material so way stronger than those cardboard puzzles!

lol i looked up the King puzzle and there are actually a couple on ebay!
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=380368521541&_rdc=1

Here's some Judy puzzles on ebay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200650503854&_rdc=1