I'm not speaking to Ravi. I've fallen _______ with him.

The phrasal verb 'fall out with someone' means to have a quarrel or disagreement with them, often resulting in not being on speaking terms. This perfectly fits the context where the speaker is not speaking to Ravi. 'Fall away', 'fall back', and 'fall through' have different meanings (decline, retreat, fail respectively). Therefore, 'out' is the correct preposition.