Dear Tina,
I am very sorry to hear about your Dad. I too have a father who has Melanoma. His started from a mole in his back to his lymph nodes. Then it went everywhere. My dad is much older than yours. He is 83, however until the melanoma spread he was an extremely active senior. He was still driving and had all of his mind working.
He is taking Temodar and Thalidomide since his cancer spread. However theThalidomide is making him very tired and weak. He is also having some problems with memory loss but it is mostly short term at this point. His personality so far is the same, however I know most of the time that as soon as the tumor gets to the frontal lobes it will affect personality in many ways. Sometimes the reason I think we see so much anger is that they are losing their metal faculities and it is so scary the easiest way to deal with it is to be angry.
I would make sure that he isn't having a medication causing some of his symptoms. Many times pain meds can make people behave in different ways or to hallucinate. Steroids commonly can make people have rages and unexplained anger especially if they are on high doses or are being brought down from a high dose to a lower dose.
I hope his personality changes get better. I agree this disease is terrible. In some ways you feel like a train is coming at you and the closer the train gets the faster it seems to go. Faster and Faster and Faster. All you can do sometimes is be there, because you never know when he is going to have that moment that he is like you Dad again.