Now on to the techniques to use: you're looking for behavior modification techniques. You can look that up if you like. It's so well documented today that you for sure won't get many strange results on the Google websites.
The first thing is very sweet. It will get you in the spirit of behavior modification. I want you to have a handkerchief that gets put in his coat pocket, pant pocket, or lunch when you take him to preschool. The handkerchief is something you and he will cuddle with when he's on your lap or next to you on the couch at home. You'll use it in story time about a boy who's so sad when his mother has to leave in the morning and he has to stay in the school she took him to. But he has this handkerchief his mom left with him to remind him she will always be back at the right time to pick him up and give him a big smile and kiss and go home happy again. And you kiss the handkerchief and he kisses it and you hug it and he hugs it. Etc. Get the idea? This handkerchief IS your love for him. And you do this every night for a while. And it goes into his pocket. Keep repeating this until it's a ritual. Then when you get to school he takes it out and you kiss it goodbye and he kisses it, then it goes back in his pocket. And then you kiss him goodbye and then you leave. And after a few nights you discuss with him if maybe the pretend boy would like his teacher to hold on to the handkerchief in her desk each morning until he leaves in the afternoon. And maybe the pretend aide can put it in the teacher's special place on/in her desk. (If all this is practical.) You begin including them as your surrogates while you're gone. So that's a sweet and important ritual to set up for him so that he begins to know there is constancy in the world. He doesn't have to have anxiety about mom disappearing. She comes back. Okay?
Please note that I'm not hesitant to let him be young. He's six but he's dealing with trauma he doesn't really remember in cognitive terms, only emotional terms. And it's in the present for him. That's stretching his developmental abilities, even though he's a bright little boy. So, don't worry about my treating him young, that's okay.
That one is very sweet and loving. Next, practice all the usual ones for regular separation anxiety: a stable schedule as much as possible (again constancy in the world); continually reassure him of your love each morning and when he comes home from school; reward him for any slight improvement (SMALL rewards, but very important); discuss at home each evening what fun thing he did at school that day. To vary the above routine, you play with him with some of his little plastic figures (animals okay) going to school, how the little boy (animal) cries that mommy leaves, but knows he's going to have a good day because he will...then supply the nice thing that happened that day...and how he knows mommy will be back to pick him up for a fun evening with supper. In other words, start rehearsing having a good time at school, having a nice time seeing mommy, having a nice evening home. You insert each time you play more of how the little boy cries less when mommy leaves. Oh, and you also don't make a big scene when you leave him in the morning. You remind him of the game you played and how much you are going to enjoy coming to pick him up in the afternoon so you can go home and play some more. Then you leave and breathe deeply and prepare your strategy for the evening. Okay?
So, follow these procedures. See if there are other ways to involve the teachers as well. And at home, let him be young.
Now, here is the web address for Psychology Today's therapist directory. You can sort by zip codes and when you see someone who seems like they might be helpful (they show you a photo of the therapist!) look at the listing and see if they list children as their focus population and play therapy or sandtray (a good method). Interview the therapist and make sure his/her values are similar to you and you feel confident and comfortable with him/her.
Good Therapy is a non profit directory. Same idea as the one above:
Okay, I wish you the very best!
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