Wow, you almost described me! I retreat when I am stressed too. I am an adult now, but in high school I was almost non functioning. My parents wanted to avoid embarrassment (my mother actually said, "If I take you to a psychiatrist, what will the neighbors think?) so they just left me alone, ignored me. They didn't know what to do with me. It is good that you are taking such a part in your son's life, you WANT to help him. He is very fortunate to have you.
I somehow managed to graduate from high school and even get a job. I am even, believe it or not, married to an NT who does not understand me, but does accept me. He gives me my space when I need it and he helps me with the social stuff.
Maybe your son feels crowded by your attention. I don't know, when I retreat to the safety of my own world, I don't like people coming around at all. I just want to be left alone. The more you push, the more I retreat.
My husband has learned how to handle me during these time (yesterday was one).
He gives me space. every once in a while he will come in to me, sit down (without touching me) and start to talk to me about one of my "special topics" (stuff that really, really interests me). He does not pressure me to talk about what is troubling me, or anything, he just listens to me.
Sometimes it takes a lot of time to get to that point, other times it takes no time at all.
He gives me a safe place to just be without any pressure. The world is a dark, scary, noisy, confusing place to me and a lot of the time I don't like it at all. He gives me a place where I don't have to conform to any societal standard, I can just be me, talk like me and act like me. He doesn't think I am strange or odd, just unique and special and extraordinary.
What I am saying is that he empowers me and lets me know that even though I don't really fit in with the rest of the world, I fit in when I am with him and it is more than OK to be me, it is wonderful. I don't feel like a misfit most of the time with him.
Maybe your son needs a haven, a place to hide from the world and relax but still have power. I wish you could talk to my husband, I bet he could help you a lot! LOL
Maybe you don't need to reach your son, maybe he needs to space to reach out to you. I don't know...that worked for me.
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I want all my senses engaged. Let me absorb the world's variety and uniqueness.
Maya Angelou