The best thing you can do is to help him develop very specific daily habits based on memorized routines. It takes those of us with ADHD longer to develop those habits; it’s typical for children to lag behind by about three years. So expect it to take three years longer of constant coaching for him to make it a habit. And even then, they’ll have to work at keeping it a habit.
A few tips:
Don’t scold or criticize. Just tell your child what they should do, gently redirect them, and patiently explain in as few words as possible what he has to do.
Don’t expect perfection. Don’t even expect tidiness. If he can find his shoes and have space to walk through the room, you are ahead. Focus on keeping things actually clean, i.e., clean sheets and no food trash.
Learn to shut the door of his room and let it be. You can’t constantly be cleaning up after him.
Get rid of extra stuff. People with ADHD tend to accumulate stuff. Have frequent clear-outs.
Give simple directions. People with ADHD can have a hard time remembering and following multiple steps. Especially children. My son was a teenager before he could be reliably expected to be thoroughly washed when he finished a bath.