I wouldn’t look it as “earning the right”. He merely needs to demonstrate that he’s capable of teaching to our syllabus.
I had an ex Royal Marine too who thought he could beast the cadets around the training area in the same way that he’d been beasted during training. He required repeated “re-education”.
Being ex service may give him the subject matter knowledge, but it doesn’t automatically make him ready to teach it, and particularly when it’s to cadets.
However if he’s the right type of guy then it shouldn’t be difficult to get him cleared to teach (except for the current issue around fieldcraft qualifications).