Our kid has been going through some fallback in school scores recently, and as a result, I've been monitoring and helping him to do his homework.

Usually, everything goes smoothly, but sometimes, when he faces an exercise he doesn't understand at first, he gets terribly frustrated and tries to evade doing it as much as possible, even if I try to help, constantly arguing that "it's too difficult", or even that I'm wrong (he is also in the mood of thinking he knows everything and his parents don't know a thing, so maybe it might hurt him when he is proven otherwise?).

I've considered the possibility that he would be just trying to "play me" to stop doing his homework, but I'm not fully convinced. It really seems to me that once he gets frustrated, he gets into a blocked state and all that remains is to be stubborn and drop the homework.

What would be the best approach to go in such a situation?

Thanks in advance

EDIT: he is 13yo. He changed schools about 1 year ago, and yes, homework has been getting harder proggresively throughout the year, but this has been ongoing and pretty consistent. Talking by heart... maybe once every week.

EDIT May 8th: Thanks all for the answers. I really do appreciate all of them. I will take very good note of them, and try to applying in the best way possible to overcome this challenge.