I have ADHD and I am a very messy person, so I 100% understand the messiness problem. Until very recently (and I'm 28 now), I did the same thing with tossing things on the floor, in closets, etc.

I don't mean to be offensive at all when I say this because this is a genuine question, but do you think that his intellectual disability may complicate the problem? I say this because maybe you need a mental health professional (i.e., therapist) to work with both you and him at the same time. Basically, family therapy sessions. The person would know quite a bit about intellectual disabilities and how to work with them effectively, and they can give your son very specific/tailored coping techniques.

I do want to share some of what has worked for me, though:

(1.) Purchase and use organization items. For example, bookshelves, plastic bins, wicker baskets, stackable shelves, plastic drawers, etc.. These things can range from very cheap ($5 or less) to more than $100, so it depends on what you like. I've found that the more organization items I have, the harder it is for me to be messy. (And truthfully, you can get cheap, 4-tier bookcases at Target for $50 or less. They don't hold a lot of weight, but if you're just storing video games or movies or light things, then it doesn't really matter.) The more compartments an organization item has, the better for me.

(2.) As the saying goes: you can't help someone who doesn't want to be helped. Thus, if you tell him to be clean but he doesn't want to be clean, then he will not be clean. However, if he WANTS to be clean, then he will be clean. So, you will need to help him want the cleanliness. (For me personally, I started having difficulties finding things in my apartment, so I wanted more organization so that I could find things easier. It also enables me to invite others over without embarrassment.)

(3.) Set small, but achievable goals. For example, everyday, pick up 10 items off the floor and put them where they belong. Also, make sure to work in one area at a time. My therapist told me that if you work in too many different spaces at once, you may get frustrated and discouraged because it may take you longer to organize everything than it would take if you just focused on one area at a time -- for example, starting with the closet, then moving to under the bed once the closet is done.