As an update to the thread, after borrowing smilingcat's Jett we knew we were in the right general size and shape. DS was happier with a cut-out. Most saddles of the right shape and size were still pretty pricey, though.

I checked Amazon and happened upon this Ritchey saddle. It's an older version of a saddle that they actually market for cyclocross--perfect! The nose is a little shorter than what's on most road bike saddles, which is too long for a smallish kid on a tiny bike (his frame is probably comparable to a 40cm CX bike). AND it's <$30 WITH shipping on Amazon.

Thanks for all the help, ladies. I'm going to have to get more videos of the happy monkey bombing around on his bike. He's been out 2x. The first time he was really nervous and not entirely happy. Yesterday was like a 180. He was squealing and zipping around and saying how much he loves his bike. He's even thinking about doing a CX race in <4 weeks.