He's 17 now and would be 18 if he graduates with his class. He has plans to graduate early and leave on his mission as soon as he can. Although he is rethinking the mission. So may enroll early at whatever school he picks.
You're right though that he would be much younger than most Stanford LDS singles. But if he only plays one year that may be of no concern. He'll be plenty busy.
If he thinks he's a one and done and off to the NBA then Duke is the better choice.