My 4 yr old is beginning to hate his father, how can I make things better for everyone?

I am a very attentive mother for my 2 kids and my world revolves around them. Their father is not so attentive and has been an alcoholic (sometimes mean) since forever. I think my 4 yr old is starting to see the fact that it's hard to get his attention alot, and the kid time his dad spends is mostly with our 8 month old. I know he loves his dad, but at the same time, I can see a resentment starting to grow in my son. It mostly started when he started seeing his dad drunk more, yelling at me for one reason or another. What can I do to prevent any further resentment he would feel in his heart for his father? I know in the end, it'll sadden my son as he grows up and I don't want him to feel for his father the same resentment I felt for mine. What can I do?