He has to pay when he books, @J62. Received wisdom on this forum (and also my own personal experience) says do not block dates for someone who won't commit to booking, who is "thinking about it" or "needs to get paid". It seems like the nice thing to do, but you are likely to end up with no booking at all at that time. Best to tell anyone wanting you to do this that you are happy for them to book, but unfortunately it is first-come-first-served. If he is serious, he will book, and not merely ask you to hold his dates.
Pre-approving is great, though. That doesn't hurt anything, and helps your response time metrics.