I would encourage him/her to wear glasses by telling him/her side-effects of not wearing glasses. Glasses will improve his/her vision and this benefit would indeed motivate him/her to wear his/her glasses. Glasses is one of the must-have accessories.You can also give him/her a stylish frame as a Birthday present. He/she might get fascinated by glasses then. 

P.s.:- I read a article 
somewhere few months back. This answer might have some similarities with that article.