1) To encourage him tell him the importance of glasses if a person has weak eyes and too tell him the consequences if he'll not wear glasses. Explain him these things politely and in a convincing manner. He will definitely understand.
2) The things you'll tell him which are mentioned above, also ask him to compare the situations - when he'll not be wearing glasses and if he'll take immediate precaution ( wearing glasses ). Your tone should be convincing then he'll get convinced too.