Hi Lee,
I am a left handed in everyday life, I shoot left handed, am a right handed golfer ambidextrous pistol shooter and combinations of both at other things.
I have always shot right handed rifles with no problem, as a left handed person you just have to get on with it.
Is he left handed at anything else or just shooting?
If so he it is more likely his eye dominance, probably left in his case.
My wife is right handed in everything except shooting, she is left eye dominant and also rather a good shot.
I would not try and force him to shoot right handed, imagine if someone made you shoot left handed, you would struggle.
Have a look here https://www.shootinguk.co.uk/shooting/instruction/issues-with-eye-dominance-15475 its more shotgun but gives relevant information.
regards
John