Do you really think half of his estate would be left to a child who he can't bear to be left alone with? One who he doesn't see as his financial responsibility right now? One who he is happy for you to go into debt to look after instead of pay for, right now?
Peel back the scales from your eyes, please, even though it's going to be painful.
Start thinking about the things that you don't want to think about.
The what if scenarios.
Your own retirement.
Why are you thinking about what other people would think if you left him, ahead of what YOUR needs are, and what works for YOU?
He isn't even meeting his son's needs. His son needs much more then a roof over his head, and that roof can be met much more cheaply than is being chosen to be at present. Is it nice? Sure I'll bet it is a nice house to live in. It's a gilded cage. Still doesn't meet yours or your DS's need for financial security, for food, water, heating, activities, hair cuts, health stuff, clothing, toys, craft stuff and childcare, does it. The gilded cage doesn't meet DS's need for a father who will look after his own son, not even for an hour. It's very clearly not going to put DS through private school. It didn't prevent you from using up your life savings on your maternity leave, in fact it caused it by being an expensive house to run that you had to pay half towards. It didn't pay your pension contributions and living expenses while you were on maternity like all decent dads do, did it.
The house is a gilded cage, your partner's asset and his weapon to beat you with. He uses it as his top trump card and to prevent you two from having a serious and honest sit down review of household finances. He has made unilateral financial decisions that have been at your medium and long term expense and to his capital gain for your entire relationship. He has gone from the generous boyfriend who will buy a round of drinks to the selfish father who has no qualms about his life partner having to use her capital that should have been invested in property, to survive while looking after his newborn son. He has been happy to let you get into debt with day to day living expenses including that of his son's and even some of his daughter's, rather than prevent that by providing even a small regular stipend towards the cost of his son. During this cost of living crisis he has paid well into his own mortgage and pension, goodness knows what his savings are like, and is paying £whatever for his daughter, including but I imagine not limited to private school fees. Meanwhile he gives you "BuT i PuT a RoOf OvEr YoUr HeAdS yOu UnGrAtEfUl WeNcH" then silent treatment if you ask for more than the occasional grocery shop.
Yes, you have allowed yourself to be financially and emotionally abused. You have been unpaid nanny and housekeeper to enable this walker to live a life of his choosing - in fact you aren't even unpaid, you have been paying for the privilege of providing home cooked food, a shopping, cooking, cleaning and child RAISING service, through paying half of the house bills, providing your own car and fuel to ferry his DD around, and providing everything from clothes and food to entertainment and education costs to his son! He's got it fucking made with you as his nanny/housekeeper, he even presumably gets sex out of it and what does it cost him for all of this? Literally fuck all, an occasional shopping trolley of food, and the occasional inconvenience of his not unpaid-but-actively-paying nanny/housekeeper/chef/handmaiden trying to have awkward conversations with him about money, which he shuts down quickly with anger then silent treatment.
He's got it absolutely MADE while you're absolutely FUCKED. Why aren't you LIVID??
You can't change the past. But you can change the future.
It starts with peeling back the scales from your eyes and seeing your situation for what it really is.
For me it then continues with you informing him that with immediate effect you're not paying towards the gas/water/electricity/council tax/household maintenance of his house or even any minor costs associated with his daughter like fuel to pick her up. You're informing him not negotiating with him since every time you try to talk about money with him he gets angry and gives you the silent treatment, but if he'd like to calmly sit down and have an open and fully transparent and honest conversation with you about equitable finances as a supposed loving and respectful couple who have a child together, you'd be open to that.