Research has found that it takes on an average of 10,000 hours of practise for a student to master the piano. http://www.bulletproofmusician.com/how-many-hours-a-day-should-you-practice/
Let's take half an hour of practise a day for example: 10000 divide (0.5 x 365) = 54.8 years. In this case, it will take your son at least 54-55 years to master the art of piano, and by that time, he will be 64 years old.
I would suggest a student to practise at least 3 hours each day, without a day missed. It will take around 9 years for the child to complete his fundamental piano studies, and thus achieved grade 8 or even ATCL diploma around the age of 18. Which is quite reasonable, isn't it?
Video showing: Ailecec piano progressions from age 7.5