Ok here is my best advice. Go and get a Mealy Amazon and when you hear that one scream the Senegal screaming will sound like music to your ears.
All joking aside now there are many questions to get answers to until we can get a better idea why he is screaming.
Senegals are not known to be loud but they can be if they want. They are very much attention hogs and its there way or the highway and they will try their hardest to be the boss!!! (I think I was a parrot in a past life)
Here is #1 Rule: Do
NOT give this bird any attention when he screams!!! This is at times extremely difficult. But you must ignore it and leave the room. You may have givin him a treat before while he was screaming and now he thinks if I scream I will get a good treat.
Cover the cage while he screams then once he stops and has stayed stopped for awhile uncover him and give him some attention. If he starts again just keep repeating. This can ometimes be a slow process but this seems to work the best. I know my senegal as soon as she is covered will not make any sound at all. Banishment from the "flock" is the only punishment parrots will understand. Screaming at them will not act as a punishment.
Location of the cage will make a huge difference. My senegal is in a bedroom with a computer and another couple of pets so its quiet and a low traffic area. Makes a huge difference!!!
While you are holding your senegal and he screams try talking quietly to him. This will make him want to hear you so he should quiet down once he realizes your talking. My senegal loves talking to me the exact same time I talk to her, not that much she says is clear enough to understand yet but talking is very quiet when they do it.
I've been teaching my parrots to use a small whistle when they want my attention instead of screaming (positive reinforcement). Senegals are excellent at whistling so teach her to whistle, then when she does it you do it back. Make it lots of fun for your bird. Then if she whistles while your not near her whistle back then go to her. Eventually they will learn to whistle when they want you. Much easier on the ears then screaming.
What choice of toys do you have for your bird and do you rotate them with new ones. Maybe your bird is bored while in the cage and wants time with you instead. Senegals love wood toys, anything with a bell, and anything they can destroy (kabobs, pinatas, etc.)
What kind of diet is your bird on? Where did you get them? Have they been to a vet? There may be an underlying health issue that makes the bird scream and they may be trying to tell you something is wrong. Do you use any teflon in the house? I've heard of many birds screaming when the fumes get into the air (this is very toxic to birds) Your roomates may use teflon frying pans or anything so be very careful.
Finally I will jump on the foraging band wagon. Foraging is a natural instinct for parrots that stimulates their minds. Set up some foraging toys and such for your senegal to get treats. This can also help with problems such as feather plucking. There is a great DVD about foraging called "Captive Foraging" Great to watch!!!
So thats my advice and this goes for all birds not just Senegals. All parrots will scream a little its their instinct. You can not stop it completly, so don't waste any money on videos and books that say "end screaming completly now" This can be a slow process but things will get better. Your senegal is still very young, so start now!!! Let us know how you make out, and if your gonna kick anyone out get rid of the roomates.