Us citizen toddler travelling back to US from India with grandparents

[san2009]

hi all,
My two and a half year old son is currently in India since 2 months and am planning to bring him back here next month.I was initially planning to travel back to INDIA and bring him back here with me but I am having a leave issue.Can he travel back to US with his grandparents only without any parent and what kind of documents would be needed?he has a PIO card and my parents have a visitors visa .any help is greatly appreciated.
thanks