No child is perfect, and the "tantrum" stage can actually last until 5-6 years old. They change a lot at this age juncture.
Try the book "Your 5 Year Old" or, "Your 6 Year Old" from www.amazon.com
Anyway, my feedback is:
1) he seems to need help with "transitioning". Some kids, need more time with this than others... they do NOT like being SUDDENLY told that something else is to occur. So, some children need a LOTS of lead-time, and a forewarning, and a head's up about what is coming up "next." With my daughter, when she is engrossed in something, she does NOT like to be told to stop.... not that she is a fussy child, but that is just her personality. So "I" learned FROM her... and I KNOW that I need to adjust my way of "making" her do something... I give her a lead-time, in advance, about what the rules are or what is coming up, or what I expect of her. ie: I know you are having fun now, but in 15 minutes, I need you to start to get dressed for school. Or, if not, you will be tardy and have to get a tardy slip from the office...." or I tell her: "you can decide- do you want to be tardy or not? Mommy will take you to school when you are ready.... it's your choice....Now, I have to get dressed myself and get your brother ready.... I will be in the car waiting..." (then I DO go and get in the car with my son, and we wait.) She knows I mean business. If we are "late" then so be it.... it is a 'lesson' in life.
2) Next, I would suggest a ROUTINE with him, and more structure, maybe. "Allow" him a certain amount of time for each activity, even of his own choosing. Use an egg-timer perhaps... maybe 20-30 minutes. Then when it is time-up... let him unwind... then segue onto the "next" thing/activity/routine. THIS will help with "transition" as well. Also, kids like this, like my daughter, need time to "un-wind" from one thing to the next. A "breather" in essence.... especially when she is tired or feels "rushed." So I have learned how to "pace" my daughter, and I can totally 'read' her.... so I know how to approach her or change my tactics.
3) Some parents take their kids to school in their pajamas. I have seen that many times. The Teachers have too.
4) When he is angry... let him. Instead of "correcting" it. ie: when he is angry... tell him "If you want to yell, go in your room to do it. It is not acceptable to take it out on others. We are a family, a team. Now, go in your room, that is your safe zone..." Then if you have to, put him in his room. And when he deflates on his own, then have him come out... and talk about it. That is what we do with my daughter... we always talk about it after.... and I ask her WHY she feels like that, HOW does she feel, WHAT is wrong... and that she can tell me anything... and I will be there for her. BUT not if she is "mean."
5) Teach your son "coping skills." All children, need this... how to cope and how to problem solve. Show him other ways he can handle his moods, feelings, outbursts, hit a pillow or something....BUT THAT YOU NEED TO TALK ABOUT IT. Especially a boy... they NEED to LEARN how to communicate... and how to understand feelings.
6) Some kids get out of control/moody when they are hungry or tired.... does your son, get enough SLEEP? Or eat in the morning? This is so important... or their blood-sugar drops and they can't cope and get all moody.
For this, I don't think harsh things are necessary... I think he needs more one-on-one type of circumventing... and letting him "vent" but teaching him HOW in a constructive way. And sometimes, a child won't stop, unless they KNOW that no one will give them attention for it.
7) each child is different... CUE INTO your child... see what makes him tic and ticked-off. Then, talk with him about it... and problem solve. That is what I do with my daughter... instead of just writing her off as "another moody mood" I actually ASK her why she is acting a certain way... and her answers really enlighten me... that is how "I" learn FROM her and how to handle her, or improve things, for ALL concerned. Whenever I talk about things with my daughter, it deflates the situation and she feels better.
At other times, you literally need to tell the child "I make up the rules, not you. Now, if you don't get ready, then NO computer at all... I will lock it...I asked you nicely to get ready, respect Mommy...." Then do so & 'lock' the computer. Then let him tantrum/scream/yell and be late for school. ** A Child WILL deflate on their own...
and sometimes, when my daughter is having a hissy fit, she will actually tell me "I just need to be alone. Don't come, I'll let you know when I feel better..." and then she does.
It's all about "coping" and teaching them, and letting them, and navigating them about it.
All the best,