Hi R.,
Sorry to hear about your frustrations, I know how hard it can be. Here's a little information on the development stage your son is in right now...
At this age he is developing his solar plexus chakra, focused on creating a separate identity, thinking and problem solving, and healthy independence. Developmental tasks are to establish the ability to think for oneself, to test reality by pushing against boundaries and people, to learn to solve problems with cause and effect thinking, to express anger and other feelings, to separate from parents and be welcomed back with love, and to begin to give up thoughts of being the center of the universe. One must face ego issues of powerlessness in the victim struggle, the fear of rejection, the need for approval, and manipulative power. Working through these issues brings security, integrity, self-confidence, responsibility, and the empowerment of accountability. The solar plexus chakra governs the area of personal power, strongly influencing the adrenal glands, which are associated with stress. People often experience the most stress when they feel powerless in their lives....
Although I understand how difficult it is to deal with, it is quite normal for children to go through this stage of development. He is also going through a lot of changes in life with the new brother, you're returning to work, new classes and could need more time to adjust. Keep working with him in a gentle way...showering him with affection and letting him know he is loved even when he is acting out. Remember to keep your anger in check, especially when dealing with his anger issues. Here's book that might be worth checking out and you might want to do a little research on understanding the chakras.
Holistic Parenting: Raising Children to a New Physical, Emotional, and Spiritual Well-Being by Lynn Wiese Sneyd
I hope this helps some.