Hi Lucy
It seems like real estate agents get into these situations easily. I too inherited Charlie from a client who never really took care of him. It took a long time to gain his trust. When I got Charlie he had virtually no feathers at all he had pulled them all out. He is in excellent shape now, plays games with us, does not stop talking, singing and dancing and thinks that he is the owner of the house. I decided to get him a friend, so I acquired a second Grey Jimmy, while they do not share the same cage, and don't really get on very well with each other, they chat to each other all day, and learn new words from each other.
Some advice, when you talk to your Grey, slightly close your eyes and make soft clucking noises, he should respond. Another game that worked was eye games, close your eyes and open them and your Grey will copy you. It takes time but you will win eventually. I also bought a very good book that helped a lot "The African Grey Parrot Handbook" by Athan and Deter. It gives lots of advice on diet, grooming, speech training, behaviour and its modification etc.
Warm Regards and Good Luck let us know how it goes
Charmain