I note that the T-shirts are advertised as for 5 to 13 year olds. As 9 years is bang in the middle of that range a 'Medium' size might well be suitable.
By coincidence I've also been looking at buying a child's shirt online. It's a Sheffield Wednesday replica shirt (at the ridiculous price of £53) but the SWFC website is a little more informative when it comes to sizes, providing this chart:
http://oi67.tinypic.com/24qiiki.jpg
Given that the age range is the same (except that SWFC don't offer an 'XS' shirt for 5-6yo kids), it once again looks as if 'Medium' is what you should be looking for.