WOW, I just want to mention that you might want to take some time to sit down with the rest of the family before Shabbos and before Yom Tov to discuss with them the fact that your son might or might not participate this year. There is no way to predict whether or not he will feel the need to, whether or not he will feel the connection. It is important to allow the other children to address their feelings about this in a safe and controlled environment and probably better to be pro-active than reactive.
If you sit down as a family prior to the Yomim Noraim and discuss this allowing each child a turn to vent calmly or ask questions in an orderly fashion, you can take some control of the situation before it becomes an uncontrollable situation. Allowing each to vent their own pain and frustration is important and explaining to them that you are “new” at handling this type of situation with their sibling but you are following the guidance of experts on the subject, might help them to understand that what they think they see and feel as hefkeirus is really not.
Depending on what they ask or say, explain to them that through phone calls and emails you are discussing every issue and incident and are receiving as much support as you can get and as many answers that are available to you. Everything that you do in regard to your son is with the hope and prior experience of others to lay the foundation to turn him around and bring him back. But of course it also takes tremendous love and tefilah on your and on their part coupled with the support of the rest of the family. In addition I would also suggest that you initiate the connection method through writing. The rest of the family should stay connected with him by writing to him and leaving him notes on his bed. It could be notes of chizuk as well as notes of pure emotion and feelings such as “it makes me sad that you don’t spend any time with me or even notice me anymore” or even “why don’t you love me any more, what did I do to you?”
Some times these notes from younger siblings will wake him and shake him to some of the realities of his own behaviors. It also relieves some of the tensions building up inside the other kids and it is important to recognize their needs as well.
Hatzlocha!