Jin,
My sense is the snakes are symbolic of a transition in your dad's life. At the age of 47 he is in the full blossom of the mid-life thing, psychological as well as physical changes in life. The snakes being in the bed in which he sleeps would symbolize unconscious aspects {sleep is an unconscious experience}. Often at this stage of life there is an indecisive outlook on life which can be paralyzing {if only temporarily}. Mid-life is a time when a person begins to give thought to the past, meaning in life, the approach of death. These things can immobilize a person, causing the person to go through bouts of self doubt and even depression.

The symptoms you gave about his demeanor, being lethargic and sad after the dream {the dream isn't what caused this attitude, it merely reflects his unconscious feelings about his life} do fit with this possibility. I recommend consultation with a mental health therapist/psychologist to help him through this period in his life.

Jerry