Upvote:4
I presume you are asking because you have seen the green card listed as a document required for entry into Italy. If so, that entry refers to the international motor vehicle insurance card, and it is only required of people who are bringing a vehicle into Italy.
The other possibility is that it refers to a US permanent resident card, but that document is not generally required of people traveling to Italy.
All your son needs to enter Italy is a passport or EU national ID card. If your son is not an EU citizen, he'll need his passport and possibly, depending on his nationality, a visa.