Spina Bifida is a common birth defect. There are many variations and therefore it is hard to characterize your nephew's specific situation without more information. Regardless, it does require medical attention. I would encourage you to have your nephew follow closely with his pediatrician.
Spina bifida is an incomplete closure of the embryonic neural tube. In general, as the body develops in the mother's womb the back develops as the vertebrae form a protective ring around the spinal cord. The bone starts in front of the spinal cord and grow around it from both sides--eventually fusing in the back. In some people, there is incomplete fusion behind the spinal cord, and spina bifida occurs.
In general there are many types of spina bifida. The variants occur based on what happens to the spinal cord. If it remains in place and there are only superficial changes to the skin, this is known as spina bifida occulta and very little is needed to be done. If the spinal cord is located outside the body this is an extreme variant and requires surgery
Talk to your nephew's doctor
. While often not a serious condition, this can be something that requires surgery. Good luck!