I’m sure this has been a terrible shock to all of you. Your dad isn’t dead to the world. He is depressed. Like many people, he has worked real hard and reached a point of transition in his life. Unfortunately, the term “midlife crisis” is often made into jokes. It’s not. It’s a very real time of taking stock and asking “Is this all there is?” For some people, radical change seems to be the only way to answer the question. The enormity of that idea often leads people to depression.
There is absolutely nothing you can do or say to argue your father out of his position. What you can do is be compassionate about how confused and sad and hopeless he feels. All of you can let him know how much you love and appreciate him. Instead of trying to talk him out of his plan, let him know that you understand that he must feel really, really desperate to throw over everything he has worked so hard for. Love him. Get by your anger and love him. He is in crisis.
Talk to your family doctor about getting your dad in for an appointment. Sometimes midlife depression is the result of illness. You want to make sure he is medically okay. If so, then see if he would be willing to be treated for his depression. A combination of medication and some talk therapy is usually the best approach. From what you say, he is an honorable man. You could try asking him to give therapy an honest try for a few months before he makes a final decision. Let him know you will all be sad but will respect his decision if he still wants out.
Be there for your mother. She needs you. You and your siblings can fold her into your families’ love and help her with raising your younger brother. You’ll also need to work on coming to terms with having a different relationship with your dad. Regardless of how he is currently behaving, he has been exemplary for many years. That may have gone underground for now but it isn’t gone. If he does stay this course, you will all need to find a way to stay connected to him even though you don’t approve of his choices.
I wish you well.