Update: My husband spoke with a crisis intervention agency in Denver yesterday, and they said that there are some police officers who are trained in mental health crisis intervention and to call the police and see if someone like that could intervene. My husband called the police station in the county where our son has been living. Long story short--without a definite location, they can't do anything. If they have a location, they can send a crisis intervention officer who would evaluate him on the spot and take him to a hospital if needed.
The police officer said we should contact our cell phone carrier and see if they could track his phone for a location. My husband was able to figure out how to do that with T-Mobile, and we saw he was somewhere in downtown Denver, but moving rapidly. So for some reason, he was downtown. The next thing I knew, however, I got a text from him which said, "You're tracking my phone?" So apparently, when someone is tracked, they get some sort of notification, which is totally dumb, because if you're trying to find someone and they don't want to be found, all they have to do is turn off their phone. Also, he's already paranoid and suspicious of us as it is, so this will probably make him even more suspicious. My husband called his phone to try to talk to him; I texted him several times and asked him to please call us so we could talk. His response was, "What about?" and he didn't call (no big surprise). So I don't know what to do from here, other than to keep pleading with him to call us and to consent to come to where we live, which he refuses to do so far. He is delusional and paranoid and I am very afraid for him.
Wish I had some encouraging report to give, but this is all I have for now. I feel like I'm waiting on a timebomb to go off any moment. This is like a nightmare that just never ends.