QUOTE(dcmbarton @ Mar 28 2009, 10:47 PM)

Ignoring this comment, have you tried recording his playing? I find that problems like that are usually identified when they actually listen to themselves back and realise that they've been thumping the keys or suchlike!
I've been trying not to respond to this but it's been bothering me. I hope you didn't mean that my post had no validity!!!!

Because I could give examples of similar circumstances where children in my clases have been thought "rude" shouting out, ignoring instructions, inappropriate comments and behaviour and it's turned out to be hearing difficulties.
I've also had children acting out because that's their "role" if you like. It's much easier to mess about because that's what's expected of you than break the mould you've been cast in.
Another cause of unexpected changes in behaviour is if something's going on elsewhere in a child's life so they "act out" in unrelated situations.
It may be none of these and your suggestion of recording is (IMO) a good one. However, please don't just dismiss my comments because to your mind they are unlikely. Unlikely doesn't mean impossible. I was only suggesting that the OP consider wider issues as well as the specific one posted about.
(I may not be a private music teacher, but I know about children of this age, being a qualified primary school teacher, having taught children between the ages of 7 and 11. - Incidently puberty starts to hit around 9 or 10 and the hormones involved can in themselves change a child's personality)