Another note to self maybe - stop talking about 'the property WE will buy' - no, it's the property YOU will buy, with YOUR money, which HE has no plans to contribute to. So... It should be:
- chosen by you, to meet your needs and those of your DC, for now AND if you should split up;
- not be of a size that it would be unnecessarily expensive to run should you split- no buying a 5- bedder;
- he should be paying rent, as he isn't investing.
Now. There's nothing wrong with him wanting to keep finances separate, it's sensible in many way. But if the above points aren't being met, then he's freeloading, and you're being daft. You're compromising your nest egg to make his life easier (eg buying a bigger house than you actually need in order for his kids to have somewhere to live) while HIS money is put to best use. That's not good.
A fair compromise would be him investing the difference between eg a three and four bedder in the main house, and you drawing up a legal doc specifying who put what in. You then get to keep a small % of your capital out of the house, but own the vast majority of it. He gets a small interest in the main house, which is his 'commitment', but keeps the vast majority of his capital and puts it into a flat. You split all living costs between you, with possibly fine tuning, eg if you know that his money is going to end up being more 'profitable' to him by bringing in rental cash while he lives with you, he pays you a % of it.
A word of warning. If, once you start going into this much detail and really pinning everything down to get it as equitable as it can be, he starts throwing a strop... It's the biggest red flag you'll ever see. Take major heed if this happens.