Grammar Nazi is usually how they describe someone who corrects other people's grammar (or spelling, or other errors).
Is there any expression that shows such a person in a positive light?
I ask because the person being corrected might actually be grateful about it and want to show this Grammar Nazi in a positive light.
John Doe is a real ____ . If he hadn't pointed out all the mistakes in my résumé , everyone would think I was a fool and I'd still be looking for a job.
I'm not looking for terms implying generic smartness. This term should specifically imply John Doe is good at English (or any other language).
EDIT: This question asks for an "alternative expression" which should be "funny, slightly reproachful, but not really offensive". It accepts the answer 'Grammar police', and other top answers include 'pedant' and 'fascist'.
I'm looking for a positive term. And those answers aren't applicable here.