The car situation was well over 10 years ago (probably more like 15), driven by having an incredibly low income, high living costs (not of his making) and a vicious circle of never being able to buy a car of any decent value, or having to simply write it off due to repairs being too expensive. Not expensive cars none of them, standard Fords/Rovers/Vauxhalls etc but it's the lottery of cheap second hand cars. He has an almost new car now with a lengthy warranty which he paid for from savings (a replacement for his last car, which was similar, and which had got to a point where it's 7yr warranty ended and it made sense to trade it in). Hindsight is a lovely thing, but that situation is pretty well covered now.
There would be no consumer frenzy 
; when he moved into his home post separation, he had to purchase everything (furniture, curtains, white goods, crockery, cutlery, toys for his DC) as all he took from the marital home were clothes and personal effects. No need to buy anything additional for the new house as he has everything he needs,
I'm not going to labour over the inheritance, save to say that he's not disinheriting his DC. If he died right now (god forbid), he has £2k of savings, maybe £500 of personal effects at best, and a car. No real assets to speak of, and very little would be left after a funeral and expenses. If he bought a house with my help, and died (again, god forbid) the day after completion, it would make sense for the house to go to me - because I'd have paid the deposit/equity, which was what he meant by saying he'd leave it to me. He's not disinheriting them (there's nothing for them to inherit at present), only making sure that if I did lend him the money, and if the worst happened, that I'd get my money back! Nothing'd been done yet anyway, as without the house purchase there is no need at this point in his life to make a will.
I have already provided for my DC to receive £30k at 21. That's entirely separate to the money I have saved. So I'm not considering squandering any inheritance. Not least because my DC real inheritance is the equity in my home, which is more than 10x more than my savings. I really don't see my DC going without, whatever I do with this £25k.
I have still not decided how to proceed as I don't know enough about the legalities yet. It may be he decides to leave it for now, to establish himself further in his job, see if Brexit produces a fall in house prices, or whatever. What I do know is if I try to imagine the shoe being on the other foot...would I expect him to help me? Not as such. But if that contribution in some form (be it in cash, on an understanding of repayment, or a legal agreement, or buying a share in the house) was absolutely essential to being able to secure a property, and my partner declined to help, I would feel disappointed, and I think it would affect the relationship. Not necessarily fatally, but I think in the long term I'd feel resentful.
I'm also mindful that we want to spend the rest of our lives together (hopefully another 40 years; more realistically given health issues, perhaps 20-25). If I didn't help him now, what would happen? He would save, pay off cards, then add to savings. 5 years from now, when we'd be looking to set up home together, at best I'd think he might have saved another £10k, so £20k in total and paid everything off. Meanwhile I've paid off my mortgage entirely and (if house prices continue to rise) I've then got a house worth maybe £600k. It just seems to me that the financial gulf between us would be even wider and seemingly imsurmountable by then.