My son is nine years old. I don't know what's going on. When I give him homework, I will tell him repeatedly that he seems to have amnesia.
First, what does he usually do? Are you playing a game? While you were talking, he was thinking about something else. Another way, you can let him look at the problem and then let him explain his thinking of solving the problem. You must affirm his answer first, and then point out the mistakes, so that he will be affirmed and have the motivation to learn.