Hi Oliver & Welcome to our family. The others have given you great advice. It will take time for him to adjust, as greys don't like change. So once he is completely comfortable, and comfortable with you, he should come around. Encourage him as much as you can, by talking to him, and use the same phrases all the time. He will bond with you, and then resume talking, I'm sure!
Looking foeward to hearing more and seeing photos!