Calmly warn Caleb that you can not guarantee his toys will still be there later if he doesn't pick them up. Then, when he goes on his merry way - grab yourself a black trashbag and pick up his toys and put them "away" in the garage, a closet. See how long it takes him to realize they're gone. Then, be empathetic. "Caleb, I'm so sorry your toys are gone. What a bummer!" (paraphrased from Love & Logic classes) Then go on about your business. Remeber, empathy - not sarcasm. And be sure NOT to say "I told you so..."
You have several options: 1. Donate the toys. 2. Keep the toys for a month or so and then trade them out with other toys. Or 3. Have Caleb do a special chore to earn back one of his toys.
There are many variations of this type -- find Caleb's "currency" and let that lead you! Sometimes kids quit caring because they have too much. "Oh Well - the dog chewed my toy. I'll go play with X now."
Good Luck and let us know how it goes!