I also have a 7 1/2 year old with ADHD. He's the most willing helper in the world, not so great with execution or staying on task, but the things I have found he can do is.
Pick up toys, clothes, various items and put them where they belong.
Changes the sheets on the bed and make the bed
Wipe off the table after meals
Wipe down the sink, toilet(he's the one who 'misses' the most often) put shampoo bottles, etc away in the bathroom
Dust. He LOVES to dust(And I despise it.) So I got him a little Swiffer Duster and he prances around the house sweeping everything within reach with it. It's not a perfect job, but it's better than what would be done if it were left up to me to dust.
He also, often makes breakfast for himself and his brother(6) and sister(2) on weekend mornings which allows me and my husband another 1/2hr-hour of precious sleep. It's usually just toast with pb and banana, but he loves feeling responsible.
I wish I could let him do something like water houseplants or sweep the floors(I tried sweeping and he knocked a lamp down, and and a vase, and then we decided that we would wait til he was older for sweeping)And watering plants, all of my plants would be dead from over-watering. He's so enthusiastic.