if he is a U.S. green-card holder or citizen, he must file tax return even if he only has income from foreign country. The tax code requires the reporting of income received anywhere in the world. Therefore, subpeona his tax records might be one way to go.
My experience of dealing with China-related cases is that if you have a well-connected attorney helping you in China, you can find out any information you want. In China, the determining factors for any case is half legal and half relationship/networking.
With a business case I dealt with, our Chinese counsel told us that a few phone calls, he could find out the assets of the party we are after...simply amazing..guess, you can use China's loose legal system to your advantage.
So, try to get help from friends or family members in China if you must get the necessary information from China.