He is not flock calling and he is not screaming, he is calling for a mate and, as no potential mate ever materializes, he keeps on calling and calling and calling. Teaching him to talk or whistle, ignoring him, giving him toys, making him forage or whatever is not going to make a big difference in this behavior, he will stop for a while and then he will start again. You need to keep him to a strict solar schedule with full exposure to twilight and get him a girlfriend he likes and, in the fall, when the days get shorter, he will stop calling and pick it up again next breeding season as the days get longer but it won't last long and he won't do it constantly. My tiels don't make a peep until the spring when they call a bit and look for a good nest and, when they find it, they shut up until the next year (I don't breed them, I am opposed to it, but I allow them to go through all the motions of breeding because it's healthy for them and makes them very happy).