Yes, Sling streams can be received in China without major problem.
Remember of course that your son needs to have a good internet connection.
The Slingbox is not installed in China, (unless of course you want to stream Chinese TV to another Country).
The Slingbox is installed in the Country that you want to stream FROM.
In China you simply receive the stream from that box, either on a pc or other device.
I setup a Slingbox for my sister to use, who is also in Shanghai - works well. However, she tells me that she only gets about 800kbps-1000kbps connection speed (when the connection is good). Speed is ok for standard definition viewing. The slingbox is hosted on Verizon FiOS in the US, so upload speeds are not bad for me to her.
However, if your son is having issues with speed (or wants to view in HD), he might want to try a specialized proxy service for slingplayer traffic. (Roughly $15-20 a month - which he can also use to access websites which are blocked in China).
Have him try it first, and if he wants to try the proxy, message here again - somone on the forums can point you in the right direction.
I have a 350 installed in Honolulu and receive in Shanghai. I also use VPN (Astrill) to access blocked websites (i.e. NY Times).
To use Slingbox and VPN at the same time, I find it best to get on to your browser, connect to Slingbox and get it running BEFORE you launch your VPN.
VPN. Astrill has three modes (Open Web, Open VPN, and Stealth). Use Open Web (also the default mode). If you use Open VPN it will cause stuttered reception.