I was hunting for programs to watch with my 9 year old, that i could bear (refuse to watch most reality tv) and we decided castle seemed fine, just seen the box set it is a cert 15, feel like bad mummy now.
I suspect it is only a 15 due to the peril and death every episode, rather than on screen violence, gore, sex, etc. If you've already judged he's fine with it (and I suspect most 9yr olds would be tbh) then I don't see why you shouldn't just continue as you were. Fret not, I've never seen an episode of Castle that I can remember having content I'd be wary of.
It is following a murder detective but it's rarely gory or extreme. Most of the episodes go in the same way and the murder/body is nearly always only shown in the first 5 minutes then the rest is catching the killer.