*TrishJ, I am going to get deeply psychological here. I am not inquiring for any information about your marriage breakdown. It is none of my business, but many children feel very afraid when a marriage breaks up and often feel that it was their fault, especially if they are an only child and have no sibling to share with. Were you a single mother for a time so that your current DH seemed an intruder in your life, taking you away from concentrating only on him.
Could your son's reaction be a sudden recrudescence of those feelings of insecurity? A feeling that first his father left him and now his mother is going away - and with the man who replaced his father? He actually may not be aware that this is the source of his deep upset. He may have just found himself being overwhelmed by feelings of grief, distress and abandonment at your decision to move away from him, hence the cause of his rage and the content of the abusive language he has used to you.
What can you do? I wish I knew, but others may. Keeping in contact by post rather than email or texts, even if he replies electronically there will have to be some gap between reading your letter and replying and keep the contact up, without mentioning the disagreements, for years if necessary.