Maybe a subject or two acceleration will do the trick...or an afterschool activity like karate or music lessons, where he learns that not everything comes easily?
I talked to his teacher from last year, who's been one of the most supportive thus far and he's offered to help lobby for whatever type of acceleration we want to try. He likes the idea of a single or dual subject acceleration (math and/or language), which should provide some add'l challenge to his day. This way, everyone would be able to monitor the "social" aspect of the change without the complete grade skip (which would could be a simple transition later). He suspected -- and I agreed -- that it's easier to reverse a single subject acceleration without any feeling of failure, whereas a full grade skip gone bad would probably attract a little more negative attention.
I completely agree with you about the after-school activities.
DS has been active in martial arts for two full years now, at his own choosing, and he enjoys it. He's competed in some local competitions, winning one and getting his little buttocks pummeled in the other. To his credit, the pummeling came first and he had no (outwardly visible) hesitation when the next tourney came around (which he won).
The other sport he enjoys is tennis. The "season" is fairly short, so it's not a big burden on our schedules.
Both of the sports are purely elective -- and should his interest fade, he can opt out provided he fulfills the current season commitment.
Piano, on the other hand... was more our choosing. He actually pestered us about starting when he was four, so we took the opportunity of using it as a carrot: "Yes, son, as soon as you turn five -- if you are reaaaallllyy good, we'll let you take lessons." (Ha!)
He's done well, and if he didn't have the ability, we would not have insisted that he continue. But... he's already better at counting than his mother and reads both clefs better than I ever could.
He definitely considers the practice to be pure evil, but enjoys his lessons and, more recently, has felt pretty proud about some of the songs he's learned. Over the summer break, I caught him sitting down to play on his own several times -- purely for fun. I told him later how much I enjoyed hearing him play while I worked in another room... just as I've enjoyed listening to his mother play over the years. I didn't want to make to big a deal, but did want him to know that it brought pleasure to others in the house... I think he understood.
Between the sports & music, I think he's staying pretty well-rounded and is having quite a bit of fun, while learning some great skills.
