Well, many children hesitate to wear glasses just because they might get teased by their friends or they will not look good in them.This surely isn't going to help.Your friend has weak eyesight so he really needs to wear glasses to correct his degree.If he doesn't wears them soon it will become too bad for his eyes. And he will not be able to complete his assignments.
Firstly we need to tell him about the disadvantages.There are a lot. You can tell him that if he continues to refuse to wear glasses,his grades will become very low and as he won't be able to write what's written on the board, he could hardly get well prepared for exams without notes.His eyesight will become weaker and weaker, and then it will be incurable. He would have to wear glasses all his life.So, if he wears them now then he will not suffer problems later.Think about some other consequences also.
You can then help him to complete his leftover assignments.Ask him to go to ophthalmologist and start wearing glasses. Also tell him that there should be no hesitation or shame in wearing glasses and tell him just to ignore if he gets teased.Be patient and calm while talking.
This is it! You can add your own solutions for this problem but this is only an example.I hope it helps you in your homeworks.Thanks!