He'll ask questions only of a spirit that seems to be coherent enough to give halfway decent answers, who has mentioned the heart of the Crossing, and only after working toward appeasing them. My thought is he'd focus on:

1. Can you describe the 'woken dead' from the heart of the Crossing?

2. Why did the woken dead allow your spirit to go free?

Iiiif he gets an answer to #2 that makes sense to ask this final question, it'll be: 3. What spirits did not escape? But that's so circumstantial I don't know that it'll be relevant.