Younger brother anxiety trigger and controlling behaviour

Hi have 2 sons age 10 and 3. My 10 year old asd and is non verbal he’s never really took to my 3 year old but has always tolerated him. He’s had a few health issues lately and self harm has really escalated he now seems out of pain but wants complete control of his environment and the people in it otherwise he’s self harming slapping his face we can’t even talk in front of him. He cannot be in the same room as my 3 year old or he is closing his eyes his legs are shaking and he’s hitting himself. I was wandering if anyone has experienced this and how you went about dealing with it as It’s real life changing for our family thank you