“if they make a mature decision to begin dressing differently than they used to, but are not intending to lessen their connection to yiddishkeit”
It seems like your saying there is a connection between wearing jeans and having a lesser connection of yiddishkiet. Maybe he just wants individuality. Maybe he wants to make a decision about how he dresses instead of following the community blindly because of social norms. Dont get me wrong, there is value to what you are saying. The decision to go against the community norms may be a way to disassociate himself from the community in a public way. But why look at the jeans as an example of his falling. If hes falling in yiddishkiet it will be evident through other areas that actually have meaning. People who see a teenager in jeans and think “what a nebach” have completely missed the point of torah and yiddishkiet. Being judgemental is a terrible aveirah and its one that all of us, myself included, need to work on.