Depends. Depends a little on what he is being seen for. Usually it will be for correction of a hernia, circumsicion or some minoor surgical procedure. Usual questions I get asked are "is the surgery necessary/" "how many of these surgeries have you done? " "explain the surgery to me in detail." one must feel comfortable with the surgeon as you are entrusting your child to him/her.
Make a list. Sit down in advance (with your child if he is old enough) and make a list of questions you might have. You may want to know how the doctor decides if surgery is needed. What other choices there are, how would the surgery be done, what type of anesthesia, how long will the recovery take, and other questions. Take the list with you to visit the surgeon and write down the answers you discuss.