If there is NO ACCEPTANCE on the part of the parents and that is one of the reasons he is going off because he is hurt and angry and does not feel unconditional love, then it is very possible that he may never return. So that might be a piece of the puzzle that you don’t know.
One of the best means of return is to love your child unconditionally and to forgive. A child no matter what age they are must know that you love them no matter what. No matter what choices they make you must love them and they must know it. We each have the power of bechira and we each might make good choices or bad choices but our children are our children no matter what they choose and if we slam the door in their face or we cut ties with them we are shoving them down the path of no return. If we show them our love, our unconditional love, we are preparing a path for them to come home to. The more we try to hang on to them and force them to be like us the more they are going to tug on the rope to be free and try it their way. The more slack that you give them to test the waters they will realize that the grass is not necessarily greener on the other side and that their parents love them so much and trusted them so much that they need to return to what they know is truth and not fiction.
Once a child starts off on their dark journey all you can do is love them and pray to Hashem. It is up to Hashem to work out his relationship with that child. For whatever reason he is going through that nisayon, and you are going through yours, it is in Hashem’s hands and the two of them need to work things out. When you said “Boruch shepatroni” you put the ownership of that relationship between Hashem and your child into their hands and they have to now work out the kinks.
However, if you and other family members turn your backs on them, slam the door in their faces, or make them feel like they are not wanted c”v, then where will they return when they have worked it out? To whom will they return to? You are only convincing them that they are right to leave and build a separate life away from you and the rest of their family and friends they have known their whole lives, basically everyone and everything they know.
If you know this family I would tell you to talk to them and tell them NOT to let their pain get in the way of their Child’s pain whatever it is. Ask them to learn to separate the two. Ask them if they love their child and if they do, they must show it before they lose him forever. Tell them to forgive him and to tell him that they love him no matter what and that he should stay in touch with them. That they are his parents and he will always be their son. Tell them they should write him a note he can take with him and he can read over and over again. These are the truths that will bring him back eventually.