HELP. My autistic brother is scared of open doors

Hi! 
i have a 6 year old younger brother, he’s verbal but can’t communicate with full sentences yet, but he recently developed extreme fear of open doors, i honestly don’t know why! Me and him are always together and i make sure he’s always okay. But now i keep seeing him trying to keep all doors closed and when one is open he closes his ears and runs frantically to close it and says “ oh no oh no” or “ scared” , I'm really lost and don't know how to understand or help him.