I was 5 years old when my baby brother was born. Before that, I was an only child. Having had that experience was my main reason for wanting my children to be much closer in age... or much further apart (but I can't wait that long
A lot of what I felt and had a hard time dealing with was due to a lack of information and misunderstandings on my part... I had been begging my parents for a younger sibling for years. When I imagined a younger sibling, I imagined a *sister* about two years younger than I was at the time who I could play with fairly soon after birth.
My parents, not knowing about my expectations, didn't really get a chance to dispel the myth I had built up for myself before my brother was born. When I discovered that not only was my sibling a BOY, but he was tiny and wouldn't be able to play (what I thought of as playing) for a couple of years, I was devastated. Not only that, but I was used to getting all of my parents' attention and wasn't getting it anymore.
So... it's very hard for him too. Although he is older, he still may not be old enough to grasp exactly why you aren't giving him the attention he's used to. He definitely isn't old enough to understand why you're frustrated with him and patience really is of the essence right now.
I'm dealing with the same thing with my 21 month old dd, except in different ways. I've been so frustrated with her lately, but then I have to stop and remember that she's just a child (not in a derogatory sense) and really not much more than a baby herself. Your son is much older, but he is used to being your baby still and it's hard to let go of that. He needs you to be patient with him now more than ever before, and if he's anything like I was then he also needs you to be there for him as much as possible.
Best wishes! It's not easy for us or for them
Take care of yourself. That's the hardest thing for me to do, but it definitely has made a difference wrt how I react towards my older daughter.
love and peace.