That is a shame that someone let him watch that. Of course many can watch it and not have any ill effect but for some children, watching this type of film it can trigger off all sorts of emotions.
I think your son needs expert advice and it's good that you are seeing a consultant.
I am not a psychiatrist but can only suggest as a layman that unwanted thoughts may be helped by your son being able to relax and try and concentrate on other things that are peaceful.
Restful music being played may help. Getting him to talk about his feelings is very important even though you may hear shocking and disgusting things.
Try to help him understand the difference between make believe fantasy and real life.
I don't know if it would help him to talk about special effects in films so that he can try and differentiate between what is real and isn't.
Mr Bean is a good example of showing a child that an actor can be a loveable character in one film and then show him Blackadder where Rowan Atkinson is rather mean.
Helping him to understand that bad things in films are a place for people to view them but not to let them cross over into real life.
I think a child psychiatrist would be of great help to your son in helping understand this.
As for the person who let him see the film I hope they have been informed of the damage their irresponsibility has caused.