No one can tell you if your son will act on these threats (and they are threats). My daughter once admitted to the voices telling her to hurt me and it scared the ever living crap out of her, she asked to be hospitalized, and I don't know if that was because she thought she might listen to the voices, or because she never wanted to hear them say things like that again. She later denied she ever heard that-I don't know if she remembers.
I think you have reason to be concerned. But I will be 100% open and honest - I a have a bias. My DH was stabbed by a family member who we never would have suspected was capable of violence against him in a million years. And also had no reason that we could figure or ever heard of as to why he did it. He never really said. This kid was NOT sz, but my point is I learned in my own life that you can never really know what someone is capable of. If you some some gut instinct quietly nagging at you, if there are a lot of risk factors that lead up to a violent possibility-take it seriously. Error on the side of caution-because you don't want to live with the regret that someone get seriously hurt-or worse and you didn't listen to that instinct or act on the evidence in front of you.
This needs to be dealt with. You can't just sit around and wait and wonder and hope nothing bad happens. I know this is will not be a popular response on here, but I cannot in good conscience not say this-he needs to be evaluated by someone professionally who knows he's saying these things. Do you guys have medical power of attorney or some other legal recourse to get this done? I may be hypervigilent because of my experience-which is why I'm giving full disclosure-but I have a bad feeling about this.