Answer: He knows that life is a cycle of death and rebirth.
Explanation:
The father of the speaker in this poem must believe in the cycle of life, death and rebirth because he compares human lives to that of an orange which gets to live, make orange seeds and then get reborn when those orange seeds grow into orange trees.
In believing that human life is perpetual, he shows his belief that humans live perpetually and in likening it to oranges coming back, the method of the perpetual living is being reborn.
