Right now the grass looks greener on the other side of the fence. He thinks that by leaving you, the children and home, that life will be better. He is seeking freedom, fun and yes, even experimenting with the life of a teenager/young adult that he thinks that he missed out on. They don't think of the consequences of their actions at this time. All they want to do is run fast and far away.
I hope that I am wrong, but if he does move out, he will distance himself from you first, then the children, pets and home and his family and friends. He will see the children, but it won't be like it is today. He will whine about money and become very selfish w/money and his time w/his "past life". All he wants to do is have fun and have no responsibilities at all.
If you have not done so, start squirreling money away, purchase gift cards that you can use later. Watch your bank and credit card statements. You may even want to look over your phone bills. It's better to be prepared when they start orbiting earth in the crisis.
I hope he comes to his senses, but it sounds like he's been thinking about moving out for quite some time. Listen to what he says and if he wants you to respond, just say "I'm sorry you feel that way". He needs to figure things out for himself and right now, he looks at you as an authority figure (mom to his being a son).
kate, I am keeping you and your family in my thoughts and prayers. Continue as you have been, come here for support and advice. Just remember, he will try to push your buttons to justify him walking and feeling the way he does. Do not take that bait...leave it on the hook for him to ponder why you aren't.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.