"Hi [Name], Due to a clerical error, it seems we're going to need to ask you to re-sign this document. What times would you be available?" This should keep his focus on what dates on his calendar are open while you're speaking with him, rather than what possible reasons he will be needing to re-sign. He'll probably think nothing of it afterwards. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.