I don't have a direct answer. The table in the link is a guideline. It is a little understated IMO. http://www.nutristrategy.com/activitylist4.htm

Age, weight and physical condition all come into play.

The chart says doing the butterfly stroke burns up to about 950 calories per hour. I predict he’s about 25% above that.

The best way to figure this out for yourself (anyone, that is) is to use a heart rate monitor