Can't change what happened but i need help to make it better

[Mumof3cheeky]

Ds7 is generaly an anxious child but the last couple days have been very bad. Not much sleep and more quiet than normal. His school made a huge mistake and somehow the kids were made to watch a very disturbing video about a disease which included someone being stabbed. It's all blown up at school as parents obviously complained and it is being dealt with but im left with a very upset child who is intelligent enough to be able to express how he feels and that he struggles to not think about it but i just don't know what to say to him or how to comfort him apart from lots of cuddles. Anxiety and depression runs in our family so im very aware of things but also frightened. Any advice how to reduce the damage since i can't undo what he had seen!