Liability insurance

Hi all , I'm the mum of a severely 16 yr old autistic son , something happened yesterday that has got me thinking,  my son had a melt down and broke my rear windscreen of my car, obviously my insurance covered it I just had to pay the excess  , but what if it was someone else's car that was in his path so I was hoping to get liability insurance for him but I can't find any thing , does anyone know of any companies that provide this cover