Navy81 makes a good point. In many cases that I have seen, playing a sport actually helps the grade situation. I know my mid used the time he had away from soccer during the season very efficiently. Grades never suffered.
The rolls of KP grads are filled with those who participated in a sport...some even in two...one of my son's soccer mates, Kenny Graff, played both soccer and baseball and graduated right on time.
I would encourage your son to play the sport he loves at KP should he be accepted to attend and should he make the team. One thing about all the service academies, if you find the time demands are just too much, they certainly cannot pull your scholarship! So, you risk little in giving it a try.
Go for it!