Having written papers for my sons, I know how you feel. In Bill Clinton’s words, “Ah feel yer pain.”
But that’s okay, ’cause I know my sons will know the same frustration when they do the same for their children.
But the grade really isn’t what’s important. What is important is that your son knows you were there for him when he asked for help, and that you were capable of providing that help. That’s what he’ll remember when he’s face with the same situation twenty years from now.