Well I like you am an optimist. He has been treated with curative intent. None of us know if we are cured, but there is no point in assuming we aren't. I do have to accept that his starting with a psa of 500 is not good, and abiterone suggests the oncologists have decided to chuck everything at his cancer in the hope of knocking it out, they wouldn't have done that if it was just a pussy cat.
However as of today there is no reason for him to think or behave as if he has not been cured.
It has been commented on here before that end of treatment is a bit of a disappointment, you are no longer the centre of attention, your just a normal person.
The HT can take about a year to recover from, so fatigue, hot flushes or any other side effects, may warrant some sympathy. Also it is possible he is depressed, people are very reluctant to get treated for depression. One of my friends said if you ever get depression don't try and struggle through. Get help.
So I don't think you need to change your attitude. Any future problems need to be dealt with in the future not now.