WOW, We are first human beings. What he is doing is NORMAL for a human being. What he is doing is NORMAL for his age and what he is doing is NORMAL for a kid going through his nisayon.
This is really something you need to let go of. YOU are probably making a bigger deal of this than your other children are. WE frum people separate our children from the time they are young because we don’t want them to mingle. WE don’t want them to get involved too early and let nature take its course. WE wait till they are marriageable age and then encourage them to date for the purpose of marriage. That is what WE do. But that is OUR NORMAL because we choose it to be. We choose to follow OUR rules for OUR reasons.
YOUR son is not following ANY of OUR rules. It is foolish to expect him to follow this one. It is foolish to expect that your other children will NOT find out about this or that they have NOT seen other children who are OTD, modern or secular in mixed company. They are NOT living in a bubble. The best you can do for your family and your sanity and that of your husband is to understand what your son is going through and where his path is leading him and to explain it to the rest of the family. You need to explain it in such a way that they understand the pain you are going through and that you need their help to daven for their brother. To have him in mind when they daven, say Tehillim and even make brochos. Because it is only through the tefillos of others that and even the extra mitzvos of those who love him that will bring him home.
Answer their questions and also say that you are NOT angry at him because he is still your son, but you are very, very hurt. You are very hurt because he is your son and when he is in pain you are in pain, and you are very hurt because every time he performs an aveirah it hurts you deeply as his mother. As a parent you want only the best for your children, and that is why you are mechanech them the way you are. You are teaching them that Hashem loves them. You are teaching them the love of Torah and mitzvos. You are teaching them the love of Yiddishkeit and to live a healthy life filled with joy and simcha. What he is doing is not healthy and is harmful and you are frightened for his safety and welfare. The mitzva of Kibud Av v’Em is a huge one. The fact that he has forgotten that just shows how sick his neshoma is right now.