Disclaimer: please note that i am not attempting to blame or offend wow (or anyone else) in any way for anything with this post but I am trying to help her understand that there are deeper things going on and that if she understands that then it would help her move forward…
Wow, according to you– your son didnt want to go to karate/music/art/etc lessons when he was younger and he didnt want to switch schools even though he was being bullied. In other words, your son was fully in control over his life and what he did or didnt do when he was say 6,8,10,12 etc. So now he is 16 and is once again taking control over his life just like he is used to doing and decided that he didnt want to be frum anymore. Why are you complaining? If it is true that you allowed him to make major life decisions at a younger age and never complained about it then you should do the same thing now and be happy and supportive of his decisions now.
Wow, I believe you mentioned that you have a sister who is not Haredi (yet you are haredi), correct? you also mentioned that you havent told your parents about your son being otd, right? If both of these things are true then I have a question for you. Did you become a bal teshuva at some point in your life and your parents fought against it but you still wanted to be frum so you went against their wishes and did what you wanted? Or did you grow up haredi and your sister rebelled against it and is no longer haredi today? If the first one is true and you became frum, then perhaps you feel embarrassed by it and have to “save face” now to your family (and friends, relative, community etc). If the latter is true however, and your sister went off the derech then it would mean that there is some sort of history in your family that you would have to look into.
and finally, wow, since you have a background in psychology then you know that you cant help anyone unless they want the help. It doesnt look like your son wants the help so there is nothing you can do for him. You can however help yourself by seeing a therapist and working through the pain you are experiencing but that is all you can do (besides for loving, supporting, and accepting your son for who he is but that you already know:). Remember, you need to uncover in order to recover…
I do have a suggestion for you if you are willing to try something more radical though which I think may be able to help you understand your son a little better but I am hesitant to share it with you since its a little out of the box. let me know if you are interested though and I will def share it with you.