Well I'm no expert and certainly don't have a wealth of knowledge, but I thought I would include our experience to the responses you will get.
We neutered Riley at 6 months old. Since he was 12 weeks old, he would "mount" any dog smaller than him, male or female. As he got older it got worse, to the point where he didn't even want to play with the other dogs, just seek out any dog that would allow this behavior even for a few moments (or until i would tell him NO and get him off the other dog). It was like he was obsessed, it was a one track mind kind of thing whenever he was around other dogs. Highly embarrassing!
I talked to experienced breeders and looked up tons of information about neutering and what age to do it long before we even got Riley. The experienced breeders seemed to feel that you should not neuter until after 12 months old, they felt it affected the dogs growth if you do it any sooner. Well, with Riley's "mounting" issue and our need to board him and do daycare (both of which required neutering after a dog reaches 6 mths.) we decided to neuter him at 6 mths.
His behavior since the neuter has greatly improved. No mounting at all, much more interest in playing with other dogs. He seems a little calmer than before (he is still a spitfire by all means, but a bit calmer now) and I don't think he seems any less boyish in looks. Nor do I feel that his growth is out of line. He is not a small boy, neither were his parents so I didn't really expect it, although I know that is not the only indicator of a dog's size.
If i could have waited until after 12 mths. I probably would have, however, given his behavior and the daycare/boarding issue it wasn't going to work that way for us. I don't regret neutering earlier, in fact I'm quite glad that we did it then. He recovered very quickly and it all worked out fine.
He was not going to have that perfect Cavalier show dog look anyway, but it doesn't matter to us. We love him the way he is. He is now 9 1/2 mths. old and weighs 16 pounds. I think he looks like a boy. Good luck with your decision, do whatever you think is best for you and Colby.
(It was a bright day so he is squinting even though i shaded him, usually his eyes look bigger )
Laura (Momma to Riley, 3 yr. old male)