Hi Chery,
We can't be positive all the time.
I know I say something very obvious, but remember this when your dad is negative.
After, he'll be back to positive.
I can imagine it's hard for you to deal with. But remember that it's normal : he is sometimes very frustrated and wants things to go quicker.
I live with this situation with my ADHD : I'd sometimes like things go quicker, but it's not possible. I, we do our best to do so but we have everyone to be patient. The worst is when mom wants things to go quicker but we can't go quicker than the music, as we say in French (with my ADHD, in my country (France), I am into a very complicated legal situation for my driving licence. It would be great for things to go quicker, but we can't make them so).
The best thing you can do is asking your dad why do he feels like that.
When he says "no" for therapies, ask him why.
Listen to him for his reasons.
Like everyone, he has the right to be frustrated for his illness. It doesn't seem to, but it's a very huge change, like a storm or a typhoon, and as for him as for everyone. You can only expect for him to be frustrated. Welcome it as a normal part of the process, because
it is a normal part of the process. Let him express frustration, fears.... You'll see that he'll feel only better after that.
Time will be your friend, his friend. Love will be friend to everyone.