It may be too late to pray that he won't go down the wrong path, but it's not too late to pray that he'll change -- with God's help.
In fact, the most important thing you can do for him is to continue to pray for him. Prison is hard, and he'll face many challenges and temptations there. If he ever needed your prayers, it is now. These seven years will either make him bitter and angry and even more rebellious -- or they'll soften him and make him turn away from the path he's been following. Which will it be?
God loves your nephew, in spite of his stubbornness and sin -- and more than anything, God wants him to wake up to the harm he's been causing, both to his family and to himself. And this can happen as God works in his heart, and he turns to Jesus Christ for the forgiveness and new life he needs. Let Samuel's commitment to the people of his day become yours: "As for me, far be it from me that I should sin against the Lord by failing to pray for you" (1 Samuel 12:23).
In addition, may you and your family do everything you can to stay in touch with your nephew, letting him know you still love him and care what happens to him. Most of all, urge him to give his life to Jesus Christ, who alone can transform him and make him into the person God created him to be.