Drexel University has a math forum where they offer complex story problems on a subscription basis. The child can attempt the problem and submit it with supporting work and get an answer whether it is right or wrong complete with a Drexel student providing feedback.
They offer a trial:
http://mathforum.org/products/store/demoAccount.htmFor a nine year old, I would say perhaps you could video record him opening up a problem and working through it. They offer challenging story problems at the pre-algebra and algebra level.
Our DYS DD also got this letter and was later admitted to the DYS program, so my advice to you is to follow up with more portfolio materials or the other testing they are asking for.
Good luck.
Neato
