His desire to see his pet in death is a natural part of coping with the reality that she is no longer an active part of his life. Death poses the dilemma of abstract thinking about the unknown for all of us. Five year olds have not yet mastered abstract thinking, let alone contemplated the mystery that is life. Making the abrupt loss of Brandy into a longer concrete process can help a 5 year old adjust to the death of a loved pet. Perhaps, in his own way, your son is suggesting a funeral for Brandy.
Funerals are rituals to help the living adjust to loss.
Especially in your son's case, as there was no warning, only sudden, irreversible change. Wanting to view the dog is an attempt on his part to make it real. Continue to remain open and responsive to his adjustment. The fact that he is talking to you about it means he trusts you with these feelings. Your son, however is finding words to express himself.