Liam's thinking tree

There is a tree in the backyard. Liam calls it his thinking tree. He loves climbing up there and being by himself. Sadly, this is rough in the winter. He doesn't have as much time to himself and he is getting really fed up with his brother and sister and not having any alone time.
He wrote his dad a note yesterday. I struggled with this post because I didn't know if it would be invading his privacy to put this out there, but I asked him if I could write about it in my blog, and he gave me permission to paraphrase. He said he wanted to disappear. And that even though we seperated the boys' space in their room, Aidan annoys him and he wants his own space. He said that there's so much swirling around in his head, and sometimes he's not doing very well with it. He said that he was typing it because he's not very good at telling us.
It's been almost all I can think about all day.
I got him on a waiting list for a therapist to work on HRT therapy with him and took a break from the therapist we were going to. But right now what I feel he needs is his old therapist. So I called her this morning, and am hoping to get an appointment soon. She left her group and started her own practice which didn't take our insurance. She thought that they would have that worked out soon, so I'm hoping that she's all set with that now.
I'm scared. I want to help and just don't know how to make him feel needed, loved and safe. Babies are easy. This is tough.