Parenting

Hi ladies I'm not really sure what I'm asking here but kinda need some advise.
One of my children is being really tough on his 14 month old. This child wakes up to 8 times a night with what seem to be nightmares. The child's father has a drug addiction and is constantly yelling and screaming at the child. He will yell when the baby is making to much noise or when he's trying to change his nappy etc. Bub is just a normal toddler exploring and learning.
I've tried talking to the child's dad to be told 'it's my child I will raise bub how I want, bub is not going to be a brat'. I've now been cut from this child's life due to other issues and I'm really concerned for bubs welfare. If dad can do this in front of others what's he doing when he is alone with Bub??
Sometimes he can be so loving and gentle with hm but the majority of the time it's like Bub is an inconvenience.