I hope this helps answer your question.
First, being a diabetic predisposes him to infection as by definition, diabetics are considered immunocompromised. However, many diabetic patients undergo surgery with no complications.
The complicated part is regarding what to do with his insulin regimen. This depends on what type of insulin he takes, short or rapid-acting, intermediate or long acting. Also when is his surgery scheduled, first case of the morning, mid day, or the end of the day. The facility &/or anesthesiologist should be contacted if they haven't given you clear preoperative instructions.
Robert Pousman, DO