First maths test of class 4 and my son has decided he's 'not a maths person'

He got 14 out of 25. Not great, not a disaster. Two silly mistakes and one question he left blank because he ran out of time. But since the paper came home, he's been saying he 'isn't a maths person', the way some of the adults around him say it. He said it to his grandmother on the phone. He said it to the boy next door. I teach in a senior school and I hear sixteen year olds say this all the time. I didn't expect it at nine, after one test. How do you stop a label like this from sticking before it hardens? I'm trying not to make a big speech about it, which is my usual instinct ❤️