Hi, my son is 7 as well with ADHD. His room was also always a mess he would unplug his lamp, move his night table, legos all over, clean clothes in dirty bin,etc. On the weekend I would help him clean up but that got old fast.
We had to find something that really motivated him and he loves playing Minecraft (yes, a video game). We let him play this 30 minutes after dinner as,ling as he is showred, cleaned his room and had a decently good behavior day at school. This worked almost immediately. He keeps track of time and will have dinner, do his own shower, clean up his room (no longer that messy since he does this daily), brush teeth and then get to play his game. This has worked consistently for 3 months now so hope it continues.