I think I would start him in Book 2 but maybe try to think of a way for him to help with his sister’s Book 1 lessons. That way he would have a review of those points but not feel like he was being demoted.
If that arrangement wouldn’t work, probably your idea of moving him through the lessons in Book 1 would work. If you did the guided discovery sections and skipped some of the transcription/dictation portions, you could probably do that quick pace pretty easily in order to review. I wouldn’t try to have him do all the transcriptions and dictations moving at that pace, especially if your main purpose is to review the usage and mechanics points.