In The Road, why is the boy like a tabernacle to his dad?
Interesting question. A tabernacle in Biblical times was the name given to the portable dwelling place for the divine presence of God that was used in Old Testament times as the Israelites wandered through the desert before reaching the Promised Land. "Tabernacle" literally means "divine presence." Looking at this word in relation to the boy, it is important to identify the way in which the boy acts almost as a divine presence for his father, in particular in the way that he reminds his father of his humanity and encourages him to act with love and compassion to those they meet.
Remember that the father, thanks to the struggle for survival he has endured, risks losing his humanity through his suspicion of everybody they come across and the way that he has had to kill others to save both his own life and the life of his son. Note for example the way that the boy reminds his father of human emotion when they meet the old man by himself on the road. The father automatically thinks he is a decoy sent to trap travellers, but the boy encourages his father to empathise with this stranger:
The boy squatted and put a hand on his shoulder. He's scared, Papa. The man is scared.
He looked up the road and down. If this is an ambush he goes first he said.
The boy acts therefore as a kind of living tabernacle of what it is to be human, what it is to love and be loved. These are things that are in danger of being forgotten by the father.