Is your house pretty dark at night? You don�t leave a lot of lights on or have a lot of commotion? A bird needs 10-12 darkness a night so that their their internal clock works correctly. The screaming is a whole different issue (unless he screams only during the hours he should be covered, and then it is a case he needs to be covered).
Previous to coming to your home had he gotten get out of cage time? Did he have mental stimulation with a lot of toys rotated frequently? How high is his cage and playpen? You need this to be lower than you are. You can't train a bird unless it is lower than your level.
Is this screaming all the time or just sometimes? Parrots scream � it is natural. If it occurs all the time you have a behavioral modification routine ahead of you. He could be screaming because he needs the 10-12 hours of darkness and isn�t getting the sleep that he needs.
Unless this screaming only happens when the bird should be covered, then it is a reinforced behavioral problem because he was a bored and unhappy bird. Yelling at the bird only reinforces the problem further � any attention is better than no attention at all. Remember, all this is from previous owner, but you�re the one that has to undo the problems and it is a timely, have patience process.
First I�d get him off to an avian veterinarian often a bird screams because it is sick or uncomfortable. Ask that a chem pan be done. If the bird proves to be healthy then we need to work on why the bird is screaming. Don�t get angry, yell, hit the cage, birds are drama king/queens, they love attention anyway they can get it.
Establish why the bird is screaming keep a bird behavioral journal and jot down the time, what he is doing when he starts screaming anything pertinent. Start establishing dominance. Make sure the bird is always lower than you are, including the cage, playpen, don�t allow him on your shoulder, It will be a long time before he will be allowed on your shoulder. Start stair stepping him. Step up step down, keep your arms around waist level, and get the bird well below you.
Reinforce good behavior with a treat or new toy. Only give him toys or treat when he is good and sitting quietly. If he screams cover him or leave the room and don�t return until he has quieted down. When he is quiet again reinforce the good behavior. Make sure he has fun interactive toys and rotate them frequently.
Tell you what, work on this, and I will write a complete article on it this weekend