Check the label on a pair of the boy's pants to see what size they are. You're looking for a number between 6 and 20. This will give you an idea of what size Speedo he will wear.
Measure the boy's waist in inches.
Measure the boy's chest in inches.
Consult the Speedo size chart to select the size that best corresponds to the boy's measurements. Look under each even-numbered clothing size for the measurements that would fit the boy comfortably. For example, if he wears a size 10 in clothing, but has a 27-inch waist, choose the Large size Speedo instead of the Medium just in case.