I second chess club as a suggestion. Chess didn't go over well for my DS though because all the local clubs teach competitive chess that uses timers. And my son hated the timer.
My area has Lego Robotics clubs run by private organization that have after school activities for kids as young as 5. Look around and see if that is available. What you want is a program that mixes age groups together as he would probably quickly get bored at the level of other kids his age.
Honestly, I would just keep supplying with Lego's and other building materials. And use find activities that stretch & compliments him in other areas that he isn't so naturally inclined. Music, Art, Sports are all good things to explore.
