Q: Can't set up Family Sharing

Hi Double E,

Family Sharing can only be set up from a Mac running Mac OS X 10.10 (Yosemite) or an iOS device such as an iPhone running iOS 8. See this article for reference -

Family Sharing - Apple Support

So you could set up the Family Sharing from your son's iPad using your Apple ID. See this article for how to sign in with a different Apple ID -

Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support

Note that the  Ask to Buy feature of Family Sharing requires the person approving purchases to be using either Yosemite or iOS 8. See this article -

Request and make purchases with Ask to Buy - Apple Support

Thanks for using Apple Support Communities.

Best,

Brett L