You could get your cockatiel a friend, however there is no guarantee that it will stop screaming! If it's attached to you then it views you as it's flock member and is searching for you. Also, you cockatiel might not bond with a new bird and you could just end up with 2 screaming birds in separate cages. If you decide to get another bird to keep your current one company then you should make sure to introduce them to each other slowly and house them in separate cage before putting them together, otherwise your current male could become territorial and attack the newcomer. And if your cockatiel is indeed a male, getting a female cockatiel would be best, as the male won't view her as as much of a threat.