There are many reasons why a cockatiel screams. I can't post them all here, but I will give you some links to help you.
First, make sure nothing is scaring him. Then assuming this is a behavior issue, you have to be super diciplined and not even look at him when he screams. If he is out and he is doing this, calmly whisper to him that he cannot be out if he screams and put him in his cage and walk away. When he is quiet go back over and praise him for being good and give a treat and let him back out. This method works well with my Indian Ringneck, but you must be very quick about coming back into the room when he is good.
Here are several links for you to look at (I also have them on my website that tries to link together many help articles)
COCKATIEL SCREAMING, HAMILTON & DISTRICT BUDGERIGAR SOCIETY INC. Cockatiel Cottage: My bird is screaming! Help! What should I do?Stop The Squawk Discover these helpful tips of training parrots not to scream By Elise KaplanKeep in mind, if it is something enviornmental that is causing the screaming, no ammount of ignoring will help. He is old enough to become hormonal and want a mate (not needed!), and he could be looking for one using a contact call to see if there is one nearby. Make sure he is getting at least 12 hours of dark time (sleep) daily. This helps kick hormonal birds out of the cycle, and yes, a single male bird can and will get hormonal! Make sure that there is nothing in the cage that resembles a nest box. This means no hidey huts, paper bags, boxes, etc.
Try to see if there is a pattern of time that he is screaming. There are normal times for birds to vocalize and noise should be expected. Many will be vocal first thing in the morning, mid afternoon, around dinnertime, and right before dusk. That is the pattern here at least!
You can also teach him an alternative to screaming, you can whistle, whisper, sing, and distract him if it is contact calling. My oldest tiel has been taught to wolf whistle for attention, and knock on the cage door when he feels he needs let out. This was done by reinforcing behaviors I would rather hear/see. It takes time, so don't get discouraged.