Well you can try the "send in the bad guy routine" if you have a bad guy. Is there someone he doesn't like? Is there a least favorite person. The bad guy needs a few days he can hang around and be available all day, (like a weekend). Every time he screams ,send in the bad guy, they don't have to do anything, just stand beside his cage until he calms down and then leave. When he behaves ,you visit and take him treats/praise him,etc. When he starts the screaming, the "bad guy" shows up and stand beside the cage. It usually works in a couple of days if your very consistent. Of course you need someone he really doesn't like or want to see. Worth a try, it works for me most of the time. especially with older birds who have a favorite person they're screaming for. They usually will try a word or phase that gets attention, the trick is to respond to what you find acceptable. If you don't he'll start screaming because that works to get attention. You can teach him that screaming brings the bad guy, but you also need to reinforce good behavior and give him a way to call you where you respond.
Merlee, you need to write that one down, it works.