I felt that way when I was pregnant with my 2nd child.
First, have faith it will be okay.
Second, ALLOW your son to be a "child"... don't "pressure" him just because he will now be an "eldest" child. They still need room to grow "into" being a sibling and having a baby around. Just like you, or any Mom.
Third, Always nurture your family as a "team" and safe place no matter what... that he can come to you for ANY reason and not just be scolded. (not that you do this). That it's okay to feel like a "baby" too... even though he is not. Its ALL normal, for a new sibling to feel this way.
Fourth, he is a child... so always remember that he is not a mini-adult... as some older siblings are expected of.
Fifth- don't make him the "example" for his baby sister... don't expect him to always be PERFECT and to always do things correctly since his younger sister is "watching" and learning from him. This is not his 'job', it is the Parent's job. It is too much pressure & stress for a young child to carry on their little shoulders. They need to BE a child, first. Not the "example" for their sibling.
If he gets jealous... this is normal. Don't scold him for it. But instead... teach him how to be a "Brother" and how to CARE for his sister, but always for what HE is capable of.... his maturity will grow. But he has to be allowed to grow AND TO MAKE MISTAKES. In time, he will then be open with you, and know that you are always there for him.
Teach him that you ALL are there for each other.... even his baby sister loves him. That no matter what... THIS IS WHAT YOU ARE ALL to each other. In the long term, and as he grows into a bigger boy and young man... this is what will impact him and be instilled in him.
Teach him "empathy" too. ALL practice it together, so he feels he is not alone in all this busy-ness with a new baby in the house, and you being even busier, as the Mommy.
ALWAYS have a routine, and CONSISTENCY in his life, that YOU provide as you can... emotionally. THIS is VERY IMPORTANT FOR A BOY. You don't want him to shut-down or to close himself off. ALWAYS encourage communication, and unconditional love, to each child, as an individual.
All the best to your and your son.... have faith and always be the person he can rely on for his safety, well being, and sharing of his "new" role in your family, and even for his happy and not-so-happy moments. Always. A child NEEDS this. No mater if you are rich or poor... these things will help make a child "richer" inside and grow his confidence and happiness.
take care,
Susan