You don't sound entirely rational.
Did you have support after he assaulted you to make sense of how that fitted into the wider picture of your relationship?
Abuse is about power and control, not violence. Somebody can be subjecting you to the most appalling abuse without ever laying a finger on you. And usually people don't realise it's happening to them until it becomes very, very extreme. Sometimes they still don't, even then.
You know he's capable of physically assaulting you, does that not make you more cautious in your behaviour knowing at the back of your mind "what if this upsets him and he hits me again?"?
It would be pretty much unheard of for a man who committed physical abuse to not also be engaging in other more subtle forms of abuse - which you may not have noticed or you may have stopped noticing.
Why would your son feel lonely if he was living with you and being cared for and loved by you? Why would having half siblings make him feel lonely? Where have you got that idea from? Unless you would plan to treat them differently based on their DNA? And remind them over breakfast each day that they have different fathers?
How do you think your second child would feel about being created as a plaything for your first child?
Why are you forcing yourself to have sex when you don't want to? Why do you think that's a normal thing to do?
A broken home is one with an abusive parent where the child is always walking on eggshells, worrying about how upset his mum is, worrying it's his fault dad is nasty to her, afraid dad will seriously hurt her one day. Where the child never gets to learn what healthy relationships look or feel like.
A home free from abuse, with a stable, loving parent is a healthy home.
Amazing dads don't ever assault their child's mother. Not once, not ever.
I really think you need some support to understand what your husband has done to you and the bigger picture of his behaviour and how it's affected you. I would encourage you to go on the Freedom Programme: www.freedomprogramme.co.uk
It's information, not therapy, so you don't have to talk about yourself. They won't judge you or tell you what to do, they just have lots of really useful info they can share with you so you can change things for the better and build that great future for you and your son.