I fear if I leave him he won't bother to see DC and that will break my heart for my sons sake.
And?
The man neglects his child when he is around him.
He isn't interested in your son. Your son isn't going to grow up thinking his dad is great if he sees you in this state and what a giantic abusive bellend his father is.
And he will also, in time, learn its ok to be a giantic abusive bellend to his partner and to not pull his weight because he's never been told differently. And he will probably grow up shitting over you too because youve been so worn down by your partner over the years and he replicates his fathers behaviour unchallenged.
Is that what you want? Is that the future you really want for your son?
Add into that the financial hardship of you ultimately losing your job to become 'carer' for your workshy lazy selfish partner and what that means for your son.
How is that future looking now?
Is your sons heart still breaking over his relationship with his father? Or is he wishing things weren't so tight money wise, his dad wasnt so abusive and his mum wasn't so utterly destroyed by trying to hold it altogether because his father cant be arse to work out that children need feeding and demand attention?
Do you want to turn into your partners mother who makes piss poor excuses for why her son is a total deadbeat?
Think about it. Your partner brings nothing to your son's life. Nothing. He neglects him. Your son would be better off without him in his life.
In time you may find another partner who turns out to be the role model and hero figure to your son that this man can never be with the mentality he has. Or maybe you won't but lets face it, your son isnt losing out that way either anyway.