Training volume for age groupers?

Good Morning I have posted before and as some may know I am the parent of a 13 year old age grouper. I have a question for all of you coaches and former swimmers as his mom and I have never swam competitively? How much volume should he be swimming at his age? I limit him to three hard workouts per week at this point and one dive practice. He also takes a lesson from a great private coach once a week that is just technique oriented and is just thirty minutes long. His total yardage is probably around 15K per week. Some of the other kids his age are doing twice the volume and the subtle pressure is there. What do you suggest? Thanks Spudfin PS I would rather he study more than swim more........