Cockatiel aggression

(Christine)

I am looking after a friends cockatiel, she can no longer look after him. I have had him for a few years now I take him to visit his "mum" once a week.
she hand raised him, and he looks forward to the visits.
He has started pacing up and down in his cage continuously a few months ago and he has become progressively worse, and he also screeches continuiously as well. He has a companion cockatiel that lives in his cage with him, they get along well and don't like to be separated for too long.
Now when I take Charlie to visit his "mum" he will screech continuously with her as well, and when he is on her shoulder he bites hard. I would be grateful for any advise that might help over come this problem. I have tried everything I can think of, eg. putting toys in cage, and changing them now and then, ignoring him when he screeches and talk to him when he is good. But nothing seems to help.