Suzi-q, I do feel so sorry for you. I would hate for this situation to be mine and it is very sad.
It may hearten you to know that a young lady I work with, who is now 19 and a delightful person, left home for 6 months when she was 17. I was surprised to find this out and asked her why. Her answer was that she was a 'rebellious bitch!' at that age! For all intents and purposes, her mother said the same to her as you did to your son. After the 6 months passed, the young lady in question moved back home. Her mother didn't back down at any time but told her daughter how delighted she was to have her home.
You mention that your son spoke to his stepdad as his father had to you...well my mother always says that "The apple never falls very far from the tree" and your son will no doubt have learned this from his Dad. You are very right not to tolerate it.
Finally, it may be worth pointing out to your son that it was he who made the choice to leave. The choice you gave him was to either treat people properly or to stay with his Dad, at no point did the choice comprise of your son over your husband or vice versa.