Did he go to any daycare or preschool leading up to kindergarten (anything structured)? Because it can take a while for kids to adjust to school, and to taking/following direction.
One of mine had a hard time even just sitting at a desk all day. I remember going in for parent teacher interviews, and even though he'd been in daycare/preschool and had done really well there, the kindergarten teacher said that my child was having a hard time - which floored me. She said he wanted to keep sitting on the floor. Well, at preschool, they sat on the floor. He didn't like sitting in a chair staring at the teacher the whole time.
He did, eventually, get used to it. I just made sure he had a lot of playtime when home, and exercise.
Is your son getting to bed on time, lots of exercise, good food (not a lot of sugar), etc? For some kiddos, sitting in a classroom is really hard.
Talk to his teacher. Schedule a meeting to go in. Ask what you can do, work together. If there's a school psychologist/counselor see if you can meet with them - see if you can observe (even from the doorway) or whatever. Get examples - when he's acting out.
Screaming seems a bit extreme. If he's just upset - find out what is setting him off (triggering the behavior). If it's after snack time, before snack .. certain times of day .. afternoon vs. morning, or if certain kids are bad for him to interact with ..
Try not to get upset with your little guy, because that just makes it worse. He may be struggling, finding it too hard. I really recommend going in and getting some suggestions from the teacher.
Did he have behavioral problems before school? Have others mentioned any?
ETA - Tadpole, that's sad (about the kid who cries). I forgot that some kids might be used to naps. When I was in kindergarten, we had nap/quiet time on mats. It's all changed. My kids had stopped by then. What I might suggest then Mom is to move his bedtime up (gradually). He's probably finding day super long. Keep same bedtime at weekends, just to keep it consistent so not a big change come Monday.
We do go back and look at previous posts (I often do). That give us a bit more to go on because sometimes we can gain insight into a behavioral issue, or see a pattern.