The thing nobody tells you is that an 8-year-old's brain isn't wired to care about homework the way you do. He's not being difficult on purpose - he genuinely finds a spelling worksheet less interesting than literally anything else happening in the room. So trying to force focus through willpower alone is fighting biology. What actually shifts things is making the work itself feel less like a chore and more like a puzzle he wants to solve.
With my nephew, the game-changer was letting him have some control over *how* he does it, not whether he does it. Instead of "sit down for math," it became "do three math problems, then pick if you want to walk around, do jumping jacks, or have a snack." He got to choose the break activity, which sounds silly but made a huge difference - suddenly he wasn't resisting because he had agency. We also did the work at a standing desk one day, the kitchen table the next, moved around. Boring environment = brain checking out. The other thing: make sure there's genuinely nothing else to do. Not just "put toys away" - actually remove them from the room. A dog walking by will beat homework every time if it's an option his brain can grab onto.
One more thing that helped was being honest about how much actually needs to happen. Sometimes what teachers assign and what a kid can realistically focus on aren't the same thing. A quick chat with his teacher about what's non-negotiable versus what's "nice to have" took a lot of pressure off. Some nights we'd do half the worksheet and that was it. The resistance dropped way down once homework didn't feel like a three-hour ordeal.