They're small birds, but parakeets can squawk almost as loudly as their larger parrot relatives. Unfortunately they don't speak English, so they depend on their kind owners to determine what they're trying to say. Just as with babies who can't talk, their calls, yells and screeches can usually be deciphered.
Parakeets in the wild call to their flockmates in the morning to tell them it's time to wake up, and in the evening to gather everyone home and signal it's time for sleep. Pet parakeets instinctively practice this behavior too at these times of day. If his cage is outside or by a window, he may also call out to birds he sees and hears. This calling can be quite loud at times.
Their social nature is one of the traits that makes parakeets so endearing, and they don't like being alone. A parakeet without a cage mate considers his owner to be his mate, so when the owner leaves the room or moves out of sight, the parakeet gets lonely and sometimes even scared. It's normal for parakeets to call out in this situation, and the calls can turn to yelling if not answered quickly. Try taking the bird—without his cage—along when leaving the room, or answer his calls so he can tell where you are until you return.
Stress or Danger
It's possible that his yelling means he's in trouble and he's calling for help. His foot could be caught in a toy, a food dish may have fallen or his ladder or perch might have become dislodged. He'll scream if he needs food or water, or maybe even if there's something new in his environment. If it's a toy or new bath, for example, try introducing it slowly—just for a few minutes at first—and stay with him as he explores it. Check to see if a bug or outside critter is upsetting him. Talking to him reassuringly can help calm him down.
If the reasons for his yelling aren't apparent, he probably just wants more attention. Parakeets require individual attention every day for at least an hour, preferably more. They love to play, but not alone. If they don't have cage mates, they expect and need their owners to play with them. Attention doesn't always have to be play, however. Parakeets are curious and love to explore, so take your bird with you around the house and talk to him as you go.
The best way to stop excessive yelling and screeching is to keep it from starting. If he realizes the way to get attention is to scream, he'll do it, so the trick is to get to him before he screams. Make sure his needs are met and he isn't in danger, and give him plenty of playtime every day. Basically, he's being rewarded for not yelling, and with patience he'll get the message.