This sounds like an interesting essay to attempt. Considering that the main character, Pi Patel, has an interesting belief system combining the world's three main religions, how about focusing your thesis statement on those three? That way, your five-paragraph essay will have a perfect format of introduction, body paragraph one, body paragraph two, body paragraph three, and conclusion. For example, your thesis could read something like this: "Pi tries to find the meaning of life by combining the world's three major religions: Islam, Hinduism, and Christianity."
Your introduction should lead to this thesis (or an alternate one that you create). The first body paragraph should focus on Pi's experience with Islam and the topic sentence should reflect that. The second body paragraph should focus on Pi's experience with Hinduism and the topic sentence should reflect that. The third body paragraph should focus on Pi's experience with Christianity and the topic sentence should reflect that.
It is the conclusion of your essay where I think you could present a new idea for the reader to consider. After beginning with a restated thesis, how about suggesting that Pi has created a belief system mirroring the current New Age idea of "All One"? The following would be a perfect quotation to use in your conclusion:
Pi Patel: "So tell me, since it makes no factual difference to you and you cant' prove the question either way, which story do you prefer? Which is the better story, the story with the animals or the story without animals?
Mr. Okamoto: "That is an interesting question..."
Mr. Chiba: "The story with the animals."
Mr. Okamoto: "Yes. The story with animals is the better story."
Pi Patel: "Thank you. And so it goes with God."
In other words, no matter how one tells the story of God (through Islam, Hinduism, or Catholicism), God remains the same. God simply changes the story to fit the person or the culture. It is a very interesting concept, and a perfect one to present in your conclusion, just as Yann Martel presented it in the conclusion to the book.