I'm going through something really similar with my 5 year old. He makes it through the day in Kindergarten, but is having marathon tantrums at home. He also does it on the way home from school, sometimes just lying in the street refusing to move while screaming his head off. He has also had a major potty regression. It's a nightmare. And, he is also really sensitive.

I don't know what sets your son off, but with my son it's anytime I say no to anything. I spoke with his counselor at school who suggested I try not to use the word 'no', and try and come up with reasoning that turns him off. For example, if he asks for ice cream (the ice cream truck parks itself outside the school) I might say that I think the ice cream in the truck isn't very clean (he's totally freaked out by 'germs') and say that we should go home and have a snack there since I know the stuff at home is clean. I also, when he is at a calm moment, explain to him that when he screams like that it makes people really unhappy to be around him at that time and they find it really upsetting. I then remind him how fun he can be and how people really enjoy being around him otherwise, then ask if he thinks he can try not to scream like that. I also make a big deal when he responds without melting down or is able to stop himself from a major meltdown. I tell him how great it was and how grown up he is acting. This all works somewhat . I'm hoping it eases up soon.