My son (11 years old) always wears mismatched socks. Even when we match them in his drawer he will unmatch them (we no longer bother). I'm not sure if style has changed or he just isn't old enough to care what girls think.
Or he thinks girls like it. That he looks more cool than other boys like that.
Well hey, if that's his style, and he can find the confidence to "own" it, the right kind of girl will appreciate him for it.
(As long as it's not too outlandish of a sock combo, ha)