Maddie did isolate herself when we brought home Cooper, our other furbutt. She would sit at the edge of the room and just 'stare' at me fromt he corner of her eye. I felt horrible. But I also noticed other behavior changes. Not wanting to play, cuddle, or sleep in our bed, instead she would go under it to sleep.
I took her to the vet for a check-up and she had the start of an ear infection. With antibiotics- and LOTS of cuddling make up time, she slowly....four months!....returned to her old self.
If pouting is his only sign, I would think it is because of the new baby. Congrats! by the way!! This is such a happy time for you all, I know it is hard to have to worry about your furbaby.
I hope he adjusts soon. Best Wishes!! ((hugs))