So, today, he and I cleaned up.
I came out to the sound of a panicked kid crying -- it wound up that he'd lost one lego bit of his new Transformer, and that wasn't good, as he'd looked everywhere and couldn't find it and it was frustrating and his new toy was RUINED!
So he got hugged and calmed down; Marx's lessons on deep breathing are working, though he seems to be better about using the deep breathing when I prompt him to, for whatever reason. He and Marx will probably wind up looking for the bit later on.
But he and I sat down with his toy box, which was overflowing, and I had him triage the toys by whether or not he actually played with him: Yes, No, and Maybe. We also sorted out stuff like his drum simulator, his solar system lecturing pad, sports stuff, pool toys, and weapons (Sting, a foam sword, a bow, his armor, two lightsabers, and a wand), apart from the rest of his stuff. The "Yes" pile wound up being about half the toys, the "Maybe" a third, and the "No" one-sixth.
I put the No toys in a bucket, and shoved the Maybes into the mesh laundry basket that had been sitting up behind the TV for a while from when he hadn't been putting all his toys away.
It looks like much less of a disaster area, especially with his sports stuff (lots of balls) shoved into a separate bucket. We'll probably sort through the Maybes, pull out the cards he put in there and put them in a box, give him a few of the Duplo blocks (the useful ones) and give the rest away to charity with the No toys.
I think the toy-triage is much better than the two usual alternatives: sorting into Keep and Throw, or having the grown-ups do the sorting. I have no idea what the logic was behind his sorting of the beanie critters, but there was evidently something, and I'm not about to quarrel with his priorities.
We're going to sort through his choices, of course, and see what things are appropriate to give to charity, which ones should go to Grandma and/or the cousins, and which ones are actually things that the grown-ups want to hang onto.