Thanks all for the congrats and advice!

I ended up telling DS6 that he is going to be part of a group of kids from all over the country who, like him, also really love to learn stuff and think. I also explained briefly that it will help us work with the school so that he can start learning more in class.

He was excited about being able to pick his own password for the YS bulletin board. I do hope we can attend some of the summer events even tho we haven't even started our 6 month intro period...