It is OK if the fashion glasses with non prescription lenses or the without frames. Personally, if your kids really want to wear fashion glasses for look, you can just buy a pair of fashion glasses frames to him. my younger sister also a one of eyeglasses fans but eith perfect vision, now, she worn eyeglasses frames about one year and still with perfect vision.
Theoretically, wearing fake fashion glasses will not generate some vision problems to your child. Or the guys chase for fashion would not wear them. But you still should tell your son that don't wear the frames long time. Because, firstly, the weight the frames made is sort of burden to his nose and this may make his nose out of shape. Secondly, when he is in a habit of wearing glasses, it is difficult to take them off. As a young lady wearing glasses several years want to that maybe no people with version problems want to wear glasses, you can't imagine that how I'm regretful now that I didn't protect my eyes when my vision was very good.