Negative memories

My Son is refusing to go to school, when we get him there his teachers report there are no problems atall . He attends a specialist setting.️ I don’t drive so my mum takes us in. Two things may have happened on the journey recently the first day back after christmas he developed a bug that night and may have felt unwell during the journey.. The other is my mums car has a sensor that beeps when she gets too close to something, he doesn’t like it but is used to it happening when my mum parks at our house or school. I don’t recall it but maybe the car beeped on the journey at some point and he wasn’t prepared for it.. Any ideas how we can cope with  or overcome these memories?