Hi Mark. My son has been staying at his Dad's for a week and a half now with no contact at all. Suddenly, today, he left me a phone message threatening to slit my throat because he fucking hates me. He also called me a fucking C---!!! Is it time to call the police? This was totally unprovoked unless he heard something from one of his friends. Everyone knows I'm concerned with drugs.
I would definitely file a complaint. If you can get an officer to the house so he can hear the message -- that would be even better.
If someone you knew (other than your son) called you and threatened to set your house on fire, would you take that seriously? ...I'm guessing you would. I don't see this threat from your son as any different.
Err on the side of safety (plus you will be showing your son that threats of physical harm have legal consequences).