Math homework battle with 3rd grader - advice needed

My 8yo gets totally frustrated with math homework and starts shutting down. Like literally says "I'm stupid" when he doesn't get something right away. Today it took us 20 minutes to get through like 10 problems because he'd get one wrong and just... give up. I'm trying to stay patient but it's tough. I don't want him to think he's dumb (he's not - he's actually pretty sharp) but I also don't want to just DO the homework for him. How do you guys handle this without turning it into a fight every night? His teacher said to take breaks but that only works so much.