It is natural for your dear son to react in this manner.
he knows that in time to come he will not be your one and only. He may try even harder to get your attention when your baby is out too.
So this is what i did. I try as much as possible to spend time with my dear daughter#1 alone. doing the activities that she like with her. I read to her books which talks about siblings, new baby and so on.
Then when baby is out, I try to have one to one time with her when baby sleeps. Plus, when she sees me carry baby she is feel very left out. So I will try to hug her more. Now she is a great help at home with baby. She helps me look after baby when I need to cook dinner, she plays with her, read to her and sometimes scolds her when she is naughty.
But i must also mention that my dear daughter#1 is in k2 this year and the age gap between the two is 5 yrs.