Our minds do funny things. Remember he is trying to figure it out himself, and that means going to a part of his brain that he for so long tried to shut out. If his story is inconsistent, it may well be that he is still having a hard time remembering it. Or he is conflating multiple abusers into one. I used to know someone who in the course of trying to remember his abuse may have realized that it was his father. That possibility hit him really hard. Really, really hard. I would say that for you to support him, the details of the past don't matter as much as the effect on the present. The specifics may come or they may not. I think how important that is varies from person to person. Don't over-focus on the past if he's not ready to. Just let him share what he wants.
"As long as the child within is not allowed to become aware of what happened to him or her, a part of his or her emotional life will remain frozen . . . all appeals to love, solidarity, and compassion will be useless."
-- Alice Miller