My daughter has a 6 year old ADHD son who doesn't listen. punishment not working. Help with ideas. Grandma?

She takes things away from him. Has tried numerous things to motivate him. rewards stickers, etc. She needs to get control of him now or she will loose him. What can we do? Won't go and brush teeth when told, tells her what to do rudely, Doesn't do unless coaxed or prodded.