My suggestion would be to let your son do it, but coach him on how. And he should probably start by asking about whether there will be more Maps assignments in the second quarter. If he actually shows the teacher how the loss of two points is affecting his grade, I suspect the teacher will spend some time thinking about it. By asking about whether there will be more Maps assignments later, your son is giving the teacher an "out" to either change the grade structure or to change the way he constructs assignments. Tell him not to press for the win right then and there, but to ask the teacher for help in understanding how he can pull up his grade. Getting into the weeds isn't a big problem if he's not pressing for an instant decision.
I agree with 22B that the problem is almost certainly lack of number sense. But it's important not to lead with "you suck at number sense," but with "help me figure out how to improve my grade."
