My son goes into school no bother (once we manage to leave which can cause him to dysregulate but that’s another story). I wish he would cry but instead he dissociates so I don’t even get a second glance.
Is he’s dissociating he’s not really going in no bother, dissociation is a sign that he’s under a lot of stress and has switched off from his feelings to cope - the dysregulation you’re seeing is him being overwhelmed by his feelings, dissociation switches those feelings off so to speak.
How understanding are the school about early trauma? I’d be talking to the school about supporting his transition into school - with dissociation they need to help him get back into his body so if they can do some sensory play with him first thing eg water, sand play, music or noise making it’ll help him be more present (and able to engage in learning). By way of example my daughter has a sensory box in class with kinetic sand, squishy toys, scented lip balm, fidget spinners and sour sweets which she uses for 10 minutes every morning while the class is getting settled for the day. It’s the difference between her being able to cope or not, being in class and engaged or being tearful, anxious and upset.
I’d also be looking to reduce other sensory input, eg ask them to put his desk in a quieter part of the class, not next to a door or window, where he doesn’t have children needing to pass him to get their tray or work etc.
You may find that part of the separation anxiety is him finding school pretty full on (even if he enjoys it) and him not understanding why it feels stressful when he likes being there. I’m guessing you help him regulate his feelings, but if you’re not there it’s hard for him to know how to do that so he gets overwhelmed.
I’d also keep working on his emotional literacy, what does love feel like to him, how does he feel when he’s worried, happy etc. One thing that can work well is to have him lie on a big bit of paper and draw round his body, then draw, use pictures, feathers, buttons etc to show where he feels his feelings - eg feathery feelings in his tummy if he’s worried, sunny pictures in his heart where he feels happy, be led by him. The more he can understand his feelings the better.