Sorry - baby is now napping and i can respond properly.
I said that about a sibling, because your post sounds very, very similar to a situation in my own family. A previously placid child suddenly exhibited a huge amount of rage when a new sibling arrived, which lasted for the next decade and a half. SO MUCH family therapy later they got to the bottom of it.
Basically, when a child is that small, and they feel sad or scared or frustrated or out of control, the easiest way for them to express it is anger. These feelings are all very, very common when a new baby arrives, especially if the child feels a little pushed out, all of which is really normal. Though the toddler may act angry, the root cause of the behaviour isn't rage, it's sadness, or fear of abandonment. Unfortunately, the rage behaviours lead to punishment, not love, which sets off a vicious cycle.
The kid is so small they're probably never aware of the feelings behind their behaviour, and although they grow up and love and get used to their sibling, the angry behaviour remains, but now they have no idea why it's happening. This behaviour frustrates and upsets the parents, making the child feel like they're not loved, which makes the anger worse, and it goes on and on.
The only way to break the cycle is with communication and so, so, so much love. Ridiculous amounts of love. Texts, notes in his room, reminiscing about his baby days, praise praise praise even for daft things, talking about what he wants to talk about. Basically just love him as much as you can. He desperately needs it.
If you give up on him now, his mother, who is supposed to love him above anyone else, how is he ever going to believe he's worthy of love from anyone else? Stop calling it anger. It's hurt, left so long its gone rock hard. You mustn't give up.