I suppose you might understand the mood changes if you had gone through all of this yourself. "I don't like the person he's become". What a horrible thing to say about someone who has been through all of this, and only has you to depend on for emotional support and strength.
Cancer with Unknown Primary is one of the worst, because they don't know where it started, what to treat it with, etc.
I have Adenocarcinoma with Unknown primary and was given less than one year to live. I have made it for four. How ? Through the strong support of my husband, family and friends... no matter how terrible I was. I was bald, I was fat from steroids, I was moody, I was in pain, and I was miserable.... and my husband's consistent comment was "You are beautiful and I love you very much".
Put yourself in his shoes if you can. Maybe you will be a little more empathetic and understanding about "what he's become" and why he's arrived there.
Your email reminds me to let all of my family members know, one more time, how much I appreciated their support, unconditional love, and understanding of what I was and am going through on a daily basis.
Thank God for my family and friends who were worried about me, and thinking about me, and not worrying about themselves during the toughest time of our lives.