\So if he's putting in 30k and that's 30% of.. the deposit? Does that mean you're putting 70k in, as that's 70%? And the mortgage is 50/50.
So:
Mortage is 60% of the cost.
30% you, 30% him.
Cash in is 40% of the cost.
Of which, 28% of total house cost is put in by you (70% of the 40% cash)
12% by him (30% of the 40% cash)
So total cost goes:
You 58%
Him 42%
I don't think you should judge his 30k as 'not from him' because his family is gifting it to him - that's unfair imo. It's money from his side, not yours. It's his investment.
So you could suggest the split is 58/42.
Or you could say that it is 50/50 ownership (as that's how the mortgage will work anyhow in terms of responsibility) but that if it is sold, you get your deposit back plus 58% of any profit, and he gets his back plus 42% of any profit, should you want to operate separately.
It doesn't sound to me great that you can't talk about it, and that you don't feel like just saying ok 50/50 all round. Because if you are buying a house together, that's a huge commitment and if this is because you're in a relationship together, I'd a bit expect each of you to put in what you can and share the property equally. It isn't a business relationship, or if it is, that's going to clash with the way a love relationship should work.
Hence him walking off and not wanting to argue. Because he thinks you are feeling superior because you personally have a flat to take equity out of, your flat cost will be covered by rent so it won't actually cost you anything, you will presumably make something out of the rental (which you aren't proposing sharing) and you are denegrading his contribution by saying it's not from him.
I know why he feels bad.
I know why you want to protect your interests.
But at some point in a relationship, you share everything.
I think it feels like you may not be at this point.