I am not at present a parent but I hope i can answer with enough sensitivity. I also havent read all 600 or so previous posts. I just wanted to add that you shouldnt overlook the importance of an emotionally suppurtive yeshiva. i mean this by mainstream yeshiva and of course in a situation like this. Of course each and every sitiuation is so different but in many cases the factor that the childwanted to dress different is because the person feels that they are or want to be different (for many or few reasons). Having someone in your life thats called “rebbi” and dresses like a rabbi thats friendly and cares about the student,and the student starts to respect them -that can totally change the way they view other people who dress like their “rabbi”. I wouldnt worry about how much lemudei kodesh they learn. he has his whole life ahead of him for that, now is the time to try to get him to have a more positive attitude towards torah. (of course the rebbi should be teaching him torah when possible, and i would trust the rebbi to know when that is).
I have a place to suggest but thats not my point here. my point is that we should remember that the “system” does not fit for all and we have to decide which parts of “going off of the path”
are “dangerous” and which parts are just searching for another path.
I wish you much hatzlacha and strength. i just want to add themt my rebbi always told us to look out for the ones that dont want to fit in- we call them leaders