I homeschool and my 9 year old is very high energy and would probably be given with some sort of ADD/ADHD label if he were in public school. That being said, we're not an "un-schooling family" - my son has regular school books and yes, he has "homework."
I let him take him plenty of breaks and give him incentive to work hard and focus. For example, he's not allowed to play with his neighbor friends until he finishes all his homework. This helps motivate him. If he's having a really hard time with a particular subject, I try to be realistic and sometimes we'll move onto something else or just take a break. He does still have to do it, but I remind him that it's okay to put it aside for a little while.
My best recommendation is to be patient with your child and set realistic expectations.