D.B.
Good details from Elena B. below.
You can't ask him to do what you do not, and you can't expect him to have a skill you and your husband don't model. Children learn by example.
So, pick a weekend, and declutter/downsize. Pick a room and all 3 of you work on that together. Get free boxes from the local liquor store and print out signs that say "donate" "recycle" and "keep." The "Donate" signs can be further broken down by charity: books can go to the library if they raise money thru book sales, clothes and knick knacks can go to places like Big Brother/Big Sister (I like them because they pick up at the garage!) or a thrift shop, winter clothing could go immediately to a local immigrant assistance group (thousands of people without power for 4 months in Puerto Rico are relocating in desperation to the US mainland, and they are freezing! Same with household items, kitchen stuff, etc.), and so on. If you think you have the energy for a yard sale when the weather gets warmer, and if you have garage or basement space to store stuff, you can make a category for that as well.
Be ruthless. If you haven't used a kitchen item in a year, get rid of it. If you haven't worn a clothing item in a year, get rid of it. Never mind if maybe you'll just lose 20 pounds or if you "might need it someday" - it's causing you stress! If you loved a book but really don't have time to read it again, let someone else enjoy it. If you're motivated to read it again someday, get it through the interlibrary loan instead of buying new books.
I don't know what kind of toy cabinet your son has, but if it's just a big mess in there, get some clear storage bins that will fit on the shelves (if there are some) or get a cheap shelf unit to put inside a big closet. Label things on the end of the bin: Cars, Legos, Puzzles, etc. Get a book organizer at the office supply store so books don't fall over - sometimes one with multiple vertical dividers is better than a set of regular bookends.
In his dresser, put dividers or even shoe boxes with the lids removed. Sweat/athletic socks go in one, black socks in another, navy socks in another (use part of the lid to divide a shoe box into 2 sections. Put another box in there for underwear/boxers. Everybody should watch a video on folding shirts/sweatshirts and storing vertically so you can open a drawer and see everything you have at once (versus stacking them so you can only see what's on top). Put hooks inside the closet doors - bedroom door for belts and bathrobe and pajamas that can be worn again. Put hooks in the hall closet for backpacks, pocketbooks, scarves, etc. Put a nice basket by whatever door you use to come in and out - that holds gloves, hats, scarves that no one wants to put away because they'll be needed shortly.
Put a shoe rack in bedroom closets. Put one or at least a shoe/boot tray by the exit door. Put a hamper in your son's closet (or a mesh laundry bag on a hook inside the door). You can consider putting a towel rack on the back of the bedroom door if you don't think he can manage to get his wet towel back to the bathroom.
Get some of those no-slip hangers so that stuff hung up doesn't slide off.
Get 6 small laundry baskets, 2 of each color, so each of you has your own color. When laundry is done, it gets folded and put in the correct basket for that person, who then takes it to their room/dresser and (gasp!) puts it away! The second set of baskets remains in the laundry room. When dirty laundry comes downstairs, so do the empty laundry basket.
Everyone learns to do laundry. Everyone learns to load, run and unload the dishwasher. When my husband and I unload, we stand on opposite sides and put away the stuff that goes in the cabinets/drawers closest to us. I hand stuff to him on occasion, and he to me. It makes the job go faster. Many hands make light work.
Start taking luxuries out of your son's room once you and your husband model the right behaviors. No electronics, phones, computers or anything else until the basics are taken care of. Responsibilities before privileges.
Every day your son comes from his room to the kitchen, he should be bringing something: his trash, his recycling, his dirty laundry, etc. Nobody ever comes empty-handed!
Start a "lost & found" bin for stuff YOU have to clean up. Stray shoes, lunch boxes, athletic gear, phone, book, backpack, permission slip for the field trip, etc. Put a bunch of cardboard strips in an envelope taped to the outside: these are Job Strips like "scrub toilet" and "vacuum den" and "dust living room tables/bookshelves" and "set table" and "take out trash from bathrooms." Any time someone wants to retrieve a lost item from the bin, they have to do a job first. Stop finding his lost stuff for him and make him do the work. Once it's done, he can start to earn back electronics or trips to the mall or rides to the movies.