I am interested in whatever answers you get, because I am dealing with the same thing from my 3 year old DD - she gets whiny, demanding, screams when she doesn't get her way, the whole bit and often by the end of the day, I've had enough. I realize that being tired, and dealing with it all day, makes me less patient. I have started enforcing consequences for when she does not listen (repeatedly asking for something after I've told her no, etc.), such as time-outs, stand in the corner, taking a toy away, etc. You might need to do the same thing with your son (if you are not already) just so he can start to take you seriously and realize that there will be consequences for not listening to Mommy and not following the rules. You can also decide what few simple rules you expect them to follow and post them where they can be seen - I know they can't read yet, but somehow having it in writing helps to drive the point home. If he persists after you have warned him, he goes in time-out. If you are comfortable with having him in time-out in his room, it will give you a chance to calm down too. You may need to get right on his level, hold him by the arms, look him straight in the face, and say, "I have asked you once already to stop hitting your brother. Since you have decided not to listen to Mommy, you will need to stand in the corner for 3 minutes." (or whatever you deem appropriate). And then do it.
Sometimes I just have to take a big deep breath and count to 10 before I just snap and blurt out the first thing that pops into my head. Don't know how much this helps, but at least you know that you are not alone. A co-worker of mine with 4 kids (12 yo twins, 9 yo and 6 yo) and I were commiserating about this and she confessed that the toddler/preschool years were her least favorite - there are more issues when they start school of course, but at least you can reason with them and better expect them to cooperate. It will not last forever!