WOW, always, always acknowledge your son and his friends. Wish them a good Shabbos, a gut Yom Tov, a good Day whatever the case may be with a warm and sincere smile. They deserve no less! It is only by acting with the utmost kovod and yiras shamayim toward these children that help them turn around. Others with seichel will also greet them kal vachomer their own parents!
As far as the other children are concerned I would suggest a family meeting. Tell the children that something happened to their brother that caused him to lose his emunah and he is considered a choleh now because he is in a lot of inner pain and turmoil. His neshomas feels lost, nah v’nad almost. His neshomah is very confused and that is why the yetzer horah has more power over him. But he is the same son and brother they always had, he is just in a bad personal place right now and he needs all the love and support the family can give him as well as understanding and rachmanus. And yes everyone should daven for him to keep him safe while he goes through this dark journey.