If he can not return to US before the reentry permit expires, his green card will be void. One soulation is that he may apply for SB-1 visa in the US Consulate in Guangzhou to retrun to US to reinstate his green card validity if he has stong evidences that his illness prevent him from returning to US on time.
See these links for SB-1 visa:
http://guangzhou.usembassy-china.org.cn/sb1_visas_returning_residents.htmm
http://travel.state.gov/content/visas/english/immigrate/returning-residents.html
http://www.immihelp.com/greencard/returning-resident-visa.html
If he does not apply for SB-1 visa, he may just abandon the green card in any one of the US Consulates in China and then, apply for the B-2 visa ath the same time or later to come to US. If he wants to get the green card again, you may just sponsor him again as you did before.
