After he wore the green one a few times (which he loves, by the way), I realized that the 3x3 rib wasn't really necessary, and kind of got in the way a bit. So I just used the 6.0mm needle at the beginning for a few rows, went down to a 4.0mm needle (I wanted it a touch tighter than the 4.5mm) and finished with the 6.0mm again. I like it, he likes it (but seems to prefer the green one right now), so all in all, a good couple of days knitting. I have more of the green and black, and will probably make a few extra of these to have around - they get lost rather easily, and you can't exactly put a tag in them.
And the husband was eyeing them with that "Can I get one of those as well?" look in his eyes, so I think this will be a good stash buster project. Totally mindless knitting.
In case anyone wants to know - I cast on 100 stitches for the 2x2 rib, 150 for the 3x3. This makes for a loose fitting neckie for a 10 year old, but the child hates anything tight around the neck. I would say you could go down a few stitches if you wanted it snugger, or certainly reduce the needle size.