Enroll him in Pre-K or Preschool....
Boys... often mature slower than girls. And he is late born.
He has a TON to adjust too... which he does not seem able to yet.
And having a new baby in the home... is hard on him.
My daughter was 4... when I had my 2nd child.
I spent a TON of time on her while I was pregnant and after the baby came home. Her little brother.
I explained, in simple terms, what a baby was. What a baby does. That a baby is not like her and does not know things like she does. That a baby cries, and wakes and needs to feed from Mommy. But that she does NOT have to worry or feel 'responsible' for it, because it is Mommy's 'job.'
I explained to her that she can TELL me anything she feels, happy or grumpy feelings. Its okay. I WILL listen to her and comfort her. My Daughter was in Preschool... when I had my 2nd baby. She was 4.
So aside from just spending time with her, my eldest... I ALSO talked with her a TON. I told her explicitly.. that I do NOT expect her to suddenly 'become' older just because she is the older child.
I kept ALL my 'expectations' of her... age appropriate... and always looked out for her and her Emotional... balance. Because behaviorally, once they do act out... it means they are ALREADY having a hard time.
So try to attend to them, PRIOR to them acting out. Acting out... like that is a symptom... of what they are feeling. Inside. Having a child's heart and feelings. Not on adult terms.
A boy, especially, NEEDS to know they CAN express their 'feelings' and that it is okay... and that they are 'allowed' to... say what they feel or think. That it is OKAY.... to feel frustration and even cry, too... and to feel awkward. It is all part... of development.
I never 'expected' my Eldest, once I had a 2nd child... to therefore be 'perfect.' I told her that.... verbatim. Because otherwise, she felt 'pressure' to be an "older" child. An older sibling. But a child that age, CANNOT act perfectly like that. They are just a kid. With stress and frustrations too.
And they can say it....
I also told my girl, 'don't worry... you are ALWAYS Mommy's first baby.... and I love you... always."
Anyway, just what I did. My girl then adjusted well... to her baby brother. They are 2 peas in a pod.. .and very close.
I focused on 'them'... as a family.
I never treated my pregnancy or my 2nd baby... as "my" routine... but as a FAMILY as a whole. That my Eldest, was a PART of that... not just a satellite floating around everything I was doing, with her baby brother.
Not easy. But your son... is having a hard time adjusting to everything. And this is normal.
all the best,
Susan