Karma to you for taking in an older grey. If I were you, I would just sit and talk to him so that he can get use to being part of your family. I would place his cage in a part of the house where he can see all the activity and become accustomed to your family. Talking and offering him a few nuts and pieces of apple might help to let him know you are his friend. I don't have any experience with abused birds but kindness goes a long way. Hopefully some of our more experienced bird owners, will chime in with more helpful advice.