My heart goes out to you. Hsve you talked to his doctor about the screaming? If there are no underlying medical issues, it could any number of things. The most common culprit for a child that age to have screaming fits is a lack of routine. Next would be a change in his environment, such as a move, or change in schedule. If none of those apply, look to see if there is a pattern to the fits. If you find that he has more fits when you have company, are out in public, or a busy time of day, it may be that he is overstimulated. If that's the case, try to make sure that he has plenty of rest and quiet time throughout the day. If you find that he quickly loses interest in his toys and wants to cling to you, he may actually be bored. Try getting together with a play group or enrolling him in daycare a few days a week. And most importantly, don't forget that Mom needs a break too. If you can't get an hour a day of "me time," hire a sitter once a week and take a day (or night) off. I had a difficult child, and darned near drove myself crazy trying to handle it all by myself all the time until my mother finally bugged me into letting her babysit for me one night a week. And, my God, has it made all the difference. Just because you're a stay at home mom doesn't mean you don't need help, don't let yourself fall into that guilt trap. We ALL need help! Just remember, no matter what your child is going through, you will not be able to help him if you are not taking proper care of yourself.
Good luck!