I would second the poster who said teach him some signs, and also get him checked out medically. A combination of those two should really help stop the screaming.
That being said it's hard to know what is causing your child to scream online. There are different kinds of screams, also- like the random really loud yells, the uncontrollable fits of anger/sadness/rage/etc, and the I'm done with this screams, to name a couple.
He is at an age where he is realizing that certain sounds mean something. He hasn't and can't master which sounds mean what yet, but he has found that screaming means something, and is very effective. (Not that he is manipulating you, or anything like that, it is just that screaming is all the tools he has at his disposal at the moment. I do not suggest that you ignore him, but try and redirect, and give him ways to communicate what he needs so you can provide it)
For me, sometimes doing something very big and silly would distract my daughter enough to "break" the cry to where we could communicate again, but when she was all worked up, there was little anyone could do.
Also, at that age, she would get overstimulated by certain people. She needed to have some calm, soothing down time, and if she was around too much hepped up energy, she would pick up on it, and it would cause her to eventually have a melt down. Also, as I'm sure you know, that when you have a bad day, your child picks up on it, and also tends to have a bad day- in turn making each others day worse. It's normal, but it sucks.
Do not forget to schedule yourself some down time, also. If you are super stressed out, its hard to see solutions. Sometimes you need to have a break, in order to think clearly. Especially when you have a child who is stong willed, you need to set aside some you time. One can survive almost anything if you know there is an end in sight/there is a set period of time. While you are in this difficult phase, utilize all the support you can get in baby watching, general assistance around the house, friendly visits, whatever you can get.
Good luck, and know that this too shall pass. :)