Hello Mike and welcome to the site.
As you can see from the other posts, a Prostate Cancer diagnosis, although horrible to hear, does not always result in the worst case scenario.
Click on Trevor Boothe's avatar above and read Trevor's profile. He is one of a number of men on here diagnosed with aggressive PC and as Julie says he is still here 4 years later, albeit having had a rocky road on some of his journey.
Irun (again check his profile by clicking on his avatar) has just completed the Sahara run.
If you go to publications on this site you can download The Toolkit. This is a lot of information regarding the various treatments and their effects and it may help with forming questions when your dad goes for his next appointment.
I expect your mum goes with him but perhaps if she isn't coping well then either you or your brother could go with them too. Take a notepad and pen to write down answers to your questions. None of you will take in everything at the time so notes are handy to reflect back on and it's perfectly normal for many on here to turn up with their written questions and make a note of the answer.
We're hear to listen and to help where we can. All is not lost. Life it just wobbly at the moment but once a treatment plan is in place lives will settle down to a new "normal" for all of you.
It's good that your parents have kept you and your brother posted because trying to keep PC secret from your children causes added strain and you all need to be supporting each other right now. I can understand your brother going off his food due to stress but he (and you) need to be fit and able to help your dad and mum.
I don't know how old you and your brother are but at some stage in the (hopefully distant ) future it might be wise to bear in mind that you would be advised to get your own PSAs done and often PC follows from father to son, as it does also with a mother having had breast/ovarian cancer.
Keep strong. You'll be an enormous help to everyone else in the family but remember you need support too