You might look into a distinction sometimes made between meltdowns, which may be thought of as external (and often may be described as 'tantrums' by teachers, colleagues, parents etc.), and shutdowns, which are internal. Here I would suggest (as somebody with asperger's who is typically very good at masking his symptoms) that your character might have a shutdown. If people speak to him, he may be unable to respond, becoming non-verbal for a period - he simply may not have the bandwidth to respond even internally to what is being asked or indeed pick up on the external stimulae. He might "look through" people he deals with every day, for example. Time might be experienced in a peculiar manner. He might to others appear catatonic, sitting, or standing in one place albeit possibly rocking at points as noted above and, at maximum, perhaps doing no more than waving people away when then do so much as ask if he is ok (though he might do the same even if they know him well, are empathetic, understand the situation and his condition and act solicitously). Stimming, for me at least, comes from a different place than this kind of a shutdown, which more or less precludes it (though as I think this through, I can imagine hand flapping and the like, perhaps coming in waves, so perhaps again not). There may be some form of an internal process, looping over the same logical explorations of something he is simply not able to take in - the computer analogy is apt here since a computer may get stuck in an infinite loop: again and again he may go over something he was planning to do which he now, obviously, cannot, but he has set himself up for his day and any change to this routine might take time to process and even minor changes might, depending upon his resilience and strategies to cope with change, provoke a meltdown or simply a bout of irascibility, but this one in particular, for obvious reasons, leads him to an incapacity to absorb reality and so, to such a stuck loop. I'm not certain this process would be in any way comparable to typical forms of thinking as might be explored in third person narration, and I do not know that I could, following such a (for me, rare) event, piece it together as anything like a stream of consciousness-style passage.
I suggest he might have the presence of mind to find a toilet cubicle or corner of a room where he will unlikely be disturbed, face a wall, sit under a table, and here shutdown. People might thereby lose him for a time. It might also, however, be in a familiar space like a sofa or chair he habitually uses. 209
There he might sit in an unfamiliar position such as in the fetal position.
For the above, I am drawing on an analysis of my own experience and, in particular, one such occasion where I had something that could be understood as a shutdown. In the next paragraph I will describe the context for this episode.
I am autistic have ADHD, am perennially underemployed, write fiction which I often doubt I will be able to publish or make any money from, and, while working on a novel while living in a different country than the one I was born in, received a letter from the Student Loan Agency asking me for forms and details I felt I could not provide them with (I am not a gifted bureaucrat). I had failed to provide such details before and so, they accused me of "evasion". I felt they could break me. I was sat on the living room sofa and, after a period of time when I rocked or some such, perhaps emitting a wailing sound, I essentially lost the power of speech. My girlfriend, who understands my condition, came to once or twice to see if she could help. I could not deal with her and waived her away. Later it would all come out of me, in angry short sentences at first and then with increasing (autistic) detail. Perhaps then she could do some of the things which normally work to calm me down when I am upset but not to such a degree: take me out for a walk etc.
That's my two cents. Others' mileage may vary. All told, however, I suspect you are on the right track. Get it down, consider then asking people with autism to give feedback, but also bear in mind that however you do it as somebody who is not themselves on the spectrum, somebody may be upset that it was not written another way.
Good luck! 873