Honestly? He's not believable, and it's kind of distracting me from the other characters (they do seem a little one-dimensional at the moment, but that could just be the fault of the short descriptions). While there are child prodigies and children who are very mature for their ages, Clarence just seems a bit much. If he was, say, twelve or thirteen, I'd be fine with him (and I'd even see a bit of myself at that age in him), but at nine, he just comes off as an older person's idea of what a child prodigy is, or the way an intelligent child would believe him/herself to be. The important thing to remember when writing children, especially highly intelligent ones, is that they almost never act like mini-adults. Their brains aren't fully developed and they don't have as much life experience to shape their actions. After all, they're still children, no matter how smart and mature they can be. And even though I'm sure nine-year-olds like Clarence do exist somewhere, they just aren't believable in fiction, in my opinion.
Anyway, that's just how I see these things. I know that I used to do the same thing in my writing, and the majority of my characters are still of well above average intelligence by default. However, I've gotten much better at balancing their intelligence with their ages, which is something that just takes practice.