Gender disappointment

Anyone have another little one that when you tell them the sex of their sibling reacted badly? Our lo has been asking Santa for a baby sister for two years. It's taken three years to have a second child. And we found out yesterday it's a boy. I'm crushed as I wanted a girl too, but I know I'll adjust just like I did last time. What about our lo? We've told him it's going to be boy and he is in denial. He keeps saying it's my baby girl. Anything I can do to help him transition to having a brother and not the sister he has been wanting?