I hear you!!!
My DS is 2.5 and I too am struggling with the tantrums, the head-strongness, the sudden bursts of over-independence, the anger, the aggression etc etc.
Basically, it is the age. They are going through so many changes at this age with regards to their language, their physical abilities, their social awareness etc. This is a trying time for them and they display their frustrations very clearly.
Don't worry, it will pass. The terrible twos do not last forever and you will get your happy, funny little boy back.
Your son is having a harder time than most though because he is also dealing with the recent arrival of his little brother.
It is great that you are trying to involve him as much as you can. That is definitely the right way to go about it. Try to help him see his brother as a positive addition to his life rather than a brat who takes your attention from him.
When DS was born I tried to involve DD as much as possible.
When breastfeeding DS, I would sit on the sofa with DD and read her stories, help her draw pictures, play with her etc. I tried to turn all that time sitting down into positive time with DD.
DD helped me to bathe DS and helped me to change his nappy etc.
I also helped her to do the same things with her own dolls. Does your DS have a doll or any sort? Maybe he would like a doll, teddy or other toy that he could treat as his baby so that he could look after it like you do.
I'm sorry if you've already thought of these points, but anything that can help your DS feel loved and secure can only be a good thing.
He's changing a lot, but he will return to the lovely little boy that he is inside.