After you've read the homeschool laws in your state, you must notify your school district that your grandson will not be returning to school. Give him at least a few weeks (or more) to unwind from his unfortunate experience, and don't start a structured learning program right away.
Turn off the TV for now and put away the Gameboys. Spend lots of time outdoors, and in the library. Find books on subjects that interest him, and read to him every day. Give your grandson lots of time to explore his world and provide him with creative learning tools -- Legos, art supplies, board and card games, etc. Visit museums, farms, and parks. Enjoy concerts and plays (many of these are available at reduced costs for homeschoolers) and help him rediscover that learning is fun. When you feel he's ready for more structured learning, read " Choosing a Homeschool Curriculum."