Thank you so much for taking him in! Cockatoos are the number one given up bird precisely for the same problems poor Jasper is showing: screaming, biting and plucking -and consider yourself lucky he is not self-mutilating because lots and lots of them do in their despair, the poor things!
See, the thing with cockatoos is that the same thing that attracts most people to them is the same that makes them suffer so much in a normal household: their cuddliness and incredible reserves of love! But, like a double-edged sword, the other side of these wonderful qualities is that they are terribly, terribly needy and cannot stand been on their own - they need company 24/7/365 (and that means their body touching yours) which people with a normal lifestyle simply cannot provide.
Cockatoos imprint and love VERY deeply and, when they lose their human, their psyche is damaged the same as a child that has been abandoned by its parents would have a lifelong trauma from the experience. And this poor guy has been abandoned many times so is it a wonder that he now bites, screams all the time and pulls his feathers out in his desperation and loneliness? No, of course not.
But, the good news is that because they are so loving and needy, they are also much easier to rehabilitate than other species! I have two of them. Freddie (23 years old) came to me because his previous owners could no longer care for him and he was a screamer (a real bad one, too!), it took me ten months but he now only makes flock calls early in the am and in the evening which is normal. He also plucked his legs bare and that has improved but has not completely disappeared. On the other hand, he is the sweetest thing ever, never, ever trying to bite me and always been extremely careful to be gentle with me - he is also a ham which LOVES attention and to make people laugh. He is one of the few birds I take among people other than me and I used him for a chat I gave to one of my grandkids summer camp (4, 5 and 6 years old children) and they adored him! (The three ladies in charge took so many pictures of him to post in FB, the bulletin board and the newsletter that I am sure half New Jersey knows him by now

). The 'new' one is Linus (25 years old) which came to me recently because, again, his owners could no longer take care of him and which I was told was not a screamer but he is (he has been driving crazy this morning, as a matter of fact

). He is also a plucker and barberer and, if you look at this thread
http://www.theparrotforum.com/viewtopic ... 12&t=14446 you will see pictures of him. He has allowed some down feathers to grow on his chest and has not plucked them out so far (but it doesn't mean he won't) and we are working on his rust and screaming issues.
Please tell us what his diet, light schedule, daily routines, etc are so we can give you specific pointers on how to help him.