May ‘18 baby hates my April ‘20 baby

Hi, wondering if anyone has any advice.

I had a son May of 2018 and I was a single mom for a while until i met my fiancé, so for a long time it was just me and him. We had a miscarriage May of 2019 and just finally welcomed our rainbow baby girl 3 weeks ago and my son HATES HER. nothing helps. Not one on one time, not gifts, nothing. Not only that but he has regressed on talking, sleeping, and other normal routine bits which I expected. I just want him to be happy and love his sister 😢