I am so sorry that you and your baby are having to face this terrible ordeal. There are several things that may happen in the final stages.
There is sometimes extreme pain associated with the brain cancer, but that does not always the case. I have seen dogs with no pain at all depending where the cancer is located.
The next thing you may see are seizures, whether abscence (Tonic) or gran mal, again it depends on the area of the brain attacked. There may be dementia, Malaise, Nystagnia, loss of appetite, loss of control of bowels and incontinence. The final stages may involve any of these symptoms or none.
The cancer of the brain can be an Enigma since there are so many different variables as to what nerves and what centers the cancer attacks.
I feel the greatest guage will be the loss of appetite, loss of balance, loss of use of limbs, seizures, and the incontinence. Those signs would be the prominent. If I can help you decipher what you are seeing please feel free to ask me for help. I will keep you and your baby in my thoughts in hope that the cancer has not spread.