My parents are still acting as repositories for some of their grown children's stuff. They have a large house, so it's not an imposition. Since you're moving out of state, debelli, I think it's entirely reasonable to have your son take some of the logistical and financial burden for moving and storing his stuff. Figure out how much you can pack/move/store yourself, and tell him he needs to fit within that limit or pay for storage/packing/etc.
I entirely agree with not throwing away what looks like junk to a parents' eye. That's how my dad lost his comics collection
The inventory sounds like the best good-faith effort you can make. Would a video of the heap of stuff also be helpful?
I'd also suggest that you save things that you think would be meaningful to your son someday, even if he doesn't think it's something worth saving. Childhood toys or clothing, favorite books, a few art projects, that sort of thing. A couple years ago my parents gave me a bundle of National Geographic School Bulletins. I'd have never saved them myself, but reading through them now is like a Proustian madeline. So many of my first mental images of the world came from that little magazine! I'm making sure to save some of the magazines that my kids once loved, but now think they're too old for (Spider, Muse, American Girl) so they can have the same experience someday.