See this thread for the best possible information.
http://www.flyertalk.com/forum/china...er-thread.html
You might want to ask the mods to move your query there as those guys know everything.

However, directly from the wiki, assuming your son was travelling on a US passport, it does seem he was not eligible for 24hr multi-stop TWOV in Wuhan and the airline was correct to deny boarding.

24-Hour TWOV

24 Hour TWOV: available in most cities except Fuzhou (FOC), Yanji (YNJ), and Shenzhen (SZX)(current exclusion as of 7 August 2016), and the only option if you have more than one stop or transit in China##

Note: passengers travelling on US and Canadian passports are not permitted to use the multiple-stop transit within China if transiting through Tianjin (TSN), Weihai (WEH), Wuhan (WUH), Xi'an (XIY) or Zhengzhou (CGO).

Sadly, as eigenvictor pointed out, it is 100% the passenger's responsibility to ensure they have the correct travel documents. Neither Priceline nor China Southern is liable here, unfortunately.