John's father says the following about John's dream.
"This is a very strong dream," he said." It may eat you up."
In fact, he says the "strong dream" part twice. Strong dream could refer to the dream simply being a very vivid and real feeling dream. I don't believe that interpretation though based on how much spiritual mysticism is present in the story. I think John's father means that John's dream shows a truth about the gods and the Place of the Gods. I think that the father knows that the dream is going to consume John's thoughts and guide his actions. It will consume John (eat him up).
The "eat you up" line is also a veiled hint of danger. I believe that John's father knows that John will go east to the Place of the Gods. I also believe that John's father knows the trip might kill John. The city was likely destroyed by a nuclear holocaust, and radiation poisoning tends to "eat" the body. Hair loss, uncontrollable vomiting, lesions, weakness, and fatigue are all common symptoms. If John goes east to prove his dream real, the Place of the Gods might very much indeed eat John alive. That's why it is forbidden to go east. The place is toxic. But John discovers that enough time has past for the radiation levels to be harmless.