procedure for obtaining a visitor visa for 6 year old nephew to visit U.S.

He will need to (through /with the assistance of a parent) apply at the consulate for a B-1/B-2 visitor visa stamp. It isn't always easy to get such a visa, even for a brief visit and even for a child, and you should speak with a qualified immigration attorney to discuss how to document the things that the consulate will need to see proven before issuing a visa.

For instance, not only will you need to prove that your nephew has been invited here and will be taken care of while here (that you will feed, house and clothe him and take care of him generally), but they will also want to be convinced that he will return home at the end of the visit. Even though his parents are remaining in Jamaica, they may not be convinced that your nephew isn't coming to the U.S. to live with you permanently and this may be difficult to prove. An examination of the exact circumstances of your life, your nephew's life, and his parents' lives may reveal creative ways in which this can be proven.

Good luck!