How do you get a DNA test from someone in prison?

This is a very complicated situation. My nephew is living with my parents. While my sister was pregnant with him, the father was killed. They were not married. The undertaker said he would save DNA for 6 months. When the baby was a few months old, my mother started receiving his social security benefits. Since the deceased father's name was on the birth certificate, social security said this was enough proof. Therefore, the undertaker was never called regarding the DNA. Now almost two years later, social security has sent a letter to my mother saying it was never proven he was the father. They are going to stop benefits along with wanting a repayment of $15,000. The deceased guy's father is in prison as a felon. How do we go about getting a DNA test between the grandfather and child? Does my mother need a lawyer or to talk to a judge or what? Please only respond if you are being serious as this is a very delicate and complicated matter. Any help is GREATLY appreciated.