It is hard to give someone news that you know they will be upset over, especially when you love the person. As I see it, you have the following choices:
A. Don't say anything either way. Let his father deal with it all as you try to recover from your son's abuse of you. This will be very hard on your relationship with him in the long run as well as the short run. When you do eventually talk to him, tell him that you were too upset to talk to him about it or you couldn't figure out how to explain it to him.
B. Tell him when he is released. It will be a huge shock to him. He will be released from the rehab facility, so if he acts out, your husband is on his own with him. Your husband may just dump your son on you without asking (we have seen it done with other kids here in the past) and leave. This gives your son what he wants and tells him that making a big mess is going to get him whatever he wants. It also damages your son's relationship with his father. You will agonize over this decision until the day you tell your son about this.
C. Tell your son as soon as possible, or at least while he still has a few days left in the facility. This gives the staff time to help him process this big change and get ready for it. Tell your son that the reason is due to your job or your health (IF and only IF you actually have health issues, but with all this stress . . . ). Make sure that he is told that this is not up for debate or discussion, and that you are sorry about it, and you love him very much. If possible, take him his favorite cookie or junk food for the trip to his Dad's. Be SURE to tell staff what you are going to tell him before you do it and then right after you do it. They need to know these things. This way gives you FAR less time to agonize over telling him. You get it done much faster.
D. Let the Rehab social worker or Family Therapist tell your son DURING a phone session. Meaning that you would be on the phone and your son would be in the room with the counselor or therapist. This might help you both through the conversation. It also ensures that at least one member of staff sees his honest, in the moment reaction. This can be a good thing. If the staff cannot see it, they cannot help treat it. Or recommend that your son gets more help for it. The more people who see how scary and dangerous he can be, the better. This might give you an extra day or so to agonize, but probably not more than that.
I hope that helps lay out the options. Sometimes another person can put them all out there for you when your mind is going in circles.