Dental treatment

Hi. My son is 13 and has autism and adhd.

He is scared of the dentist but does go and I sit in the room with him and reassure him when they are looking at his teeth.

However, last visit they said he has a cavity and needs a filling,

personally I don’t think he’s going to cope, but the dentist had told me they will be fine.

Is there anything I can do to help prepare him mentally for this? I have a horrible idea in my mind that as soon as they get the drill out he’s going to run away screaming. He’s terrified of anything medical and last time he had a blood test he had a full on meltdown and screamed the hospital down.