Dear J.,
First, give yourself a big hug and pat on the back for taking your nephew in. What an honorable and emotionally charged event this must be for ALL.
I've raised 5 children, 2 are siblings I rescued from our parents and 3 of my own. They are ALL big readers, and very gifted.
Here are my suggestions for a boy:
1. Find topics he's interested in. Boys tend to love non-fiction stories, books about trains, planes, automobiles, science, nature, hurricanes, boats, etc. If you don't know yet what interests him, then plan a trip with him to the library or any bookstore and peruse the children's shelves. Let him pick out a couple of books.
2. Set up a comfortable, safe place in your home where you both can read together. I would think for the first several weeks you would need to read TO him. After re-reading some of his favorites for the umpteenth time, stop at the end of sentences and let him fill in the ending. This works really well with rhyming books...anything by Shel Silverstein would work well here, because he's mature enough to understand the humor.
3. I still read Dr. Seus and Eric Carle books to my youngest, ages 6 & 9. They're just loads of fun and we stop to practice the tongue twisters, when there not tired of course. Which brings up an important issue of time. Evening reading after dinner before bed is really about relaxing. I tend to do ALL the reading at this time of day. If I want their input, then it probably won't happen then.
4. Also, each of my children behaves differently to reading. The boys tend to fidget more, look around, but believe me, they are listening. The girls seem to sit more quietly, one insists on being right in my lap, one will ask lots of questions, one will know if I changed even one word in the story.
5. We also play Scrabble and Boggle together a lot, with our own rules, mainly we can put the words anywhere, move them anytime to accommodate more words. Even our kindergartner plays this. We just take turns and help each other out with their letters. Since spelling and sounding out is such an integral part of reading, this will undoubtedly augment his interest in reading.
Most importantly, be relaxed about it. Correct their mistakes by speaking the misspoken correctly, if that makes sense. I never berate, belittle, or say "no" that's wrong. Just say the words correctly and move on. Their little brains are taking so much in and if your little guy has experienced trauma in his early years, it will take him a bit longer to adjust. Give him time and plenty of reassurance.
If you would like specific titles of books, let me know.
Best of luck!
Jennifer