How To Help The Firstborn To Adjust To New Siblings?

Hello, this is a question based on the behavior of my almost 2 and 4 month old son. I gave birth almost six months a go to a baby girl. Since her birth, his behavior has dramatically changed. I am guessing that he s very jealous of her. My husband and I have continued giving him most of our time so he doesn t feel neglected in any way. However, nothing helps when he whines/cries for no reason. If we are out in the park he cries if he needs to go home, be changed, to eat. It seems everything is a struggle. Is this a normal behavior and is there anything one can do about it? Thank you in advance!