I think you should sign the form. Not signing will just make things hard on your son who is required to attend. So unless you have other options for schooling your child then I wouldn't make a big deal out of this for his sake.
I can kinda understand the schools position...had that been a gun brought to school and no one was notified and something did happen well we all know what we would think of the students who did know, yet did nothing about it. jmho
I agree--sign the form. You probably signed a code of conduct when the school year began which stated that a student seeing a weapon is to report it. (What would have happened to your son had the boy actually used it in a deadly way--that is why he is to report to a teacher if he sees a knife.)
The statement is not false, the administrator talked to your son about it and is making a paper trail for the expulsion--not against your child but for the expulsion hearing.
Your son has learned a lesson, and now you have too. Sometimes unpleasant things happen but we take it like a man and get on with life.