My Autistic DD is younger, but I'm in a similar pickle. She has aaaall the Funko pops, Squishmallows, etc, and like your son, throws nothing away (and keeps all the packaging)
This year, so far, I'm thinking about an Apple Watch (so she can track her steps and heart rate; she has other health conditions) and also a cassette player and bespoke Japanese table as she is obsessed with all things Japanese (anime, etc) I'm also going to buy duplicates of her favourite clothes (she will only wear black) but that doesn't seem like an actual Christmas present.
Other than that, I'm stumped as she won't go on holiday, as she prefers home. I can always get her vouchers for Minecraft etc, but they seem boring. Last year I bought her tickets to an anime expo, but she couldn't cope with it.
I'm going to start a cash ISA for her, but my goodness, this year is going to be thin on the ground for actual presents.
Edited to add that a weighted blanket didn't go down well, and neither did an Oodie.