S.B.
Home school when done right, takes a lot of discipline and time. It does not sound like you have the time to invest. And if I were your parents, I would not want to be in charge of homeschooling. That is an immense task to put on them.
I do have to admit, that I wonder, its it possible your expectations are just too high? If the teacher says he is being challenged and meeting his potential, is it possible he is where he should be and there is progression, just not great leaps and bounds. This may not be the case, but I have to ask.
I would try a few things. If he is not engaged, make it engaging when possible. My son HATES spelling practice. He despises it. It was a fight every night. Then one day I gave him dry erase markers and had him write it on the windows. Took a photo and printed the homework that way. His teacher had NO problems with it. No more fights, and his spelling grade went way up.
When I was a student teacher, I was in charge of a child with ADHD. It was an extreme case and his parents refused any sort of medication for him. One afternoon when he was especially off task, I told him to take off his shoes and socks. There was a square bucket full of sand in the room (it was also a play therapy room) and I had him put his feet in it. He rubbed his feet back and forth and made a little mess. But he wrote two whole pages that day..it was a victory.
Is it possible the schedule needs to be changed up? If he getting a break after school, maybe that needs to stop. I know my son does better if we knock out homework while he is in "school mode". If I wait too long, it's more of a challenge. If he is not getting a break, maybe he needs one? If he is on medication, maybe that needs to be looked at. Either completing work before it "wears off" or changing the meds.
And I know you said there isn't much time for you to help. Have you tried it? Maybe if he has all afternoon "off the clock", he can knock the homework out faster with you at home.
Tutoring may be an option. I would find a private tutor. Many teachers are not fans of chains like Sullivan and Oxford. They often rely on "tricks" and teach things differently than teachers, so it can sometimes become a conflict.
ANd finally, it may be time to reevaluate his IEP. If an entire homework assignment is taking him hours. It may be time to modify things...he does the odd numbers or only half the assignment. I had several students with homework modifications when I was a teacher, it is not an unusual request.