I appear to have asperger's, not autism, but before I knew anything about AS, I knew something seemed "wrong" with my communication and my social skills. I began to research expressive aphasia, because I became aware that I had a lot of trouble communicating what I wanted to say, that sometimes I would just not talk, sometimes I would say the opposite of what I meant, and sometimes I would blend all the words I was trying to say into a type of gibberish. My family learned to sort of decipher what I meant, and so it wasn't really a big issue at home, but at work it became problematic. I only do this in stressful or exciting situations, so it is not as noticeable as it could be. I also have auditory processing issues, and at one point I wondered if I was hard of hearing. My hearing is fine, but sometimes I can't easily process what people are trying to say. If I read the same information, I can process it without difficulty. So anyway, the child you are working with could have any of the above issues, or some totally different ones. Speaking slowly, communicating in writing or in pictures, and respecting his decision not to speak might be helpful. Sometimes I just can't think of anything logical or pertinent to say, so I'll often say nothing, rather than engage in small talk. This doesn't go over well socially. Also, the child might just be done with communicating for the moment, and be needing some space and quiet. People with autism spectrum usually need lots of quiet time alone.
Its great that you are working with these kids, and that you seem so caring and engaged! Thanks for your hard work!
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"The measure of a man's estimate of your strength is the kind of weapons he feels that he must use in order to hold you fast in a prescribed place." Howard Thurman