There are lots of pictures on the internet showing proper sight alignment with the target.
Here's a video--the first part is pretty good and he might get something from it, but I would stop it before it gets into the discussion of sight adjustment.
https://www.youtube.com/watch?v=FxxUPqEjkTk
But before showing him any of that, you need to get the answers to some questions. You can't get the answers by simply asking because if he's having vision troubles, he has no proper reference to work from. So you're going to have to devise some tests to get accurate answers.
Can he see the sights on the gun ok while he's in the shooting position?
Is he keeping the proper eye open while sighting?
Can he clearly see the pictures you're drawing for him?
Can he see the target he's supposed to be aiming at?
If he can see all of those things and still can't understand how to align the sights, even after looking at pictures, I don't really know where to go from there.