Let's assume for a minute that there is no love between this man and his "girlfriend". After all, they are sleeping in separate rooms and he's not engaged with his little family.
It begs the question "why did he move in?" If he's gaming and drinking all the time, then he's not there to be with his son or to co-parent. And if he's not sleeping in the same room as the mother, then he's not there for any type of relationship with her either.
I would suggest you bide your time. When this girl gets tired of having a man-child as well as a baby, she will throw him out. And then you can have him.
Be careful what you wish for.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.