Question

Whatwould you say him to cnvience him to wear glasses

Answer 1

to convince him to wear glasses, we can use "should" for simple convince and "Ought to" for emphatic convince. 

You should/ought to wear glasses.

[it it's extremely necessary for him to wear glasses, I prefer using "Had better"

You had better [short form "You'd better] wear glasses. ]

Hope it helped. ^^