If he gets 100s on all of his tests, I'd say he's performing above average, unless the tests are so easy that all the average kids score that way too.
He might not be lazy, but perfectionistic. Do you think that's possible? Essentially, he might be afraid of a challenge because he might be afraid of failing, and feeling less smart. He might also feel like an impostor-- for instance if he knows he's considered to be gifted based on his IQ scores, but doesn't feel gifted.
He's not the first gifted kid to go into stealth mode.
I might start by trying to find a spark of interest in something and kindle it into a flame. At least the current educational fit isn't bad for him, except that it's not inspiring to him.
