Autism 7th grade but IEP file doesn’t have Tourette’s syndrome on there.

Hello Behavioral Therapist for autistic children. I have a case with a 7th grade autistic child who is kind of low functioning, ie/ non-verbal-basic little bit of sign language, he’s able to write spelling5 times a day, copy down his calendar works but with semi-readable writing but sometimes doesn’t copy the sentences correctly. Point is, he makes this sound that never stops it’s a loud screaming sound that sounds like a bird kind of. So loud we have neighbors calling the school to complain. In his IEP file I went thought it and says nothing about Tourette’s but this sound is so excessive he can’t even eat. He also has gotten really aggressive with me due to me doing hand over hand to get him to work or stop the sound. How can I help him.? His mom hasn’t even taken actions but I’m afraid for his health and I think it’s part of his stim but it never stops never. Not sure how’s his voice never gets lost. Thanks.