I wouldn't make him pay for it. You don't want to turn him off of libraries and have him stop taking out books. Like you said - it was an accident. Is there any way you can cut out the damaged page and replace it?
I think Melinda's suggestion is a good one. Maybe scan the text and tape in a new page copy? I also wouldn't charge for the book.
I wouldn't charge for the book. We just have loss at times.