My mother bringing my son to USA

[radhar4]

I am in USA now.My mother will be coming to USA on visitors visa.And my son will be coming in dependant visa.My mother will be bringing my son.
Please let me know what are all the documents my mother should carry to bring my son as expected at Immigration.Is only birth certificate of my son enough?Or is there any letter of some specific format required from my end with my signature on it?Is there any other required documents?Please let me know.

[immihelp]

What's his citizenship? She needs to carry his passport and authorization from you that he can travel with grandma.