Sarah, in one word, ignore, you will have to just ignore his screaming for there is no other way. Be sure you are not reinforcing his screaming by reacting to it for he may be doing it because of what you are doing to try to make him stop.
When he starts screaming, turn your back on him and leave him alone for a bit, then when he stops for a bit then go praise him highly or give him a treat.
Some have had success with talking softly to their bird like almost whispering and see if he will lower his scream in response to your lower voice but whenever he is quiet you need to praise him highly.
Do you give him constant attention? If you do then you have created the problem, they do need some attention but should not be getting so much that he hardly spends any time alone except for sleeping. You should have only given him as much attention as you could realistically keep up for the remainder of his life.
I know this will be difficult but I think you can turn him around if you work at it and we do have some threads that deal with this sort of thing.