You need to ask a councillar aboutthis it is threatening enough to be of cocern, remove all the knives at once,ask him Why he did it,Has the 12yearold been teasing him as older siblings are apt to do and your 9year old couldnt take it.ASk questions alone with him find out what is happening what is he emotional about has there been any new Happenings , to him is someone Bullying or ridiculing him, your son needs your Help.,