So very sorry for the latest news. Prayers that all is well in October, and comfort and peace until then. A dear friend just went through the same thing, it was months before they did a second PET, I do not know the reason why it's done this way, but we trusted the doctors to know what they're doing as they do this on a daily basis.
Not talking can be a part of grief, and coming to terms with the news. It could be denial, it could be shock, it could be a feeling of being overwhelmed. Don't rush him, if he's like some people he may never want to talk about it, his choice and his way of dealing with this blow. Hopefully he will want to share his feelings soon.
And, as far as "power of attorney," it does not go into effect until and unless he is unable to communicate his wishes for his care and treatment, verbal or written, not before or until. In some cases it never becomes necessary, as the person remains in control of their mental capacities.
If he chooses to do nothing it is his wish and his choice, you have no choice but to accept it, as much as you may disagree with him, this isn't about you, it's about him. Been there, done that, it isn't easy, it just is what it is, being there for the one you love, through whatever life brings.
{{HUGS}}