Stimming

Hi, I have a 27 year old autistic son, he also has severe learning disability, quadriplegic cerebral palsy and is non verbal. For most of his life he has done a strange moment that has changed over the years as his range of movement has grown more limited with age/size. He will tense, curl his arms up, turn his head to the side and stick his tongue out. He tends to do it more when he is poorly or tired and I would say doesn't do it when he's happy and relaxed.  He can also, sometimes, be distracted from doing it. I am wondering if it could be his form of stimming, because of the severity of his physical disabilities there is very little intentional movement he can do. He also has absences, but because he can't move himself away to be quiet or alone I wonder if it is him going into himself to regulate himself. At the moment these movements are being treated, unsuccessfully, as seizure activity but I am beginning to wonder if they are related more to the autism. If anyone can share any information on stimming that might help I would be very grateful.