Oh, Yellow, this is a nasty quandary to be in. But it gives you a new perspective on your man, doesn't it?
How about thinking of your own needs, your own exhaustion from 2 jobs, & your own future, while you reappraise how you feel about all this?
You say he is never a shirker or tight with money
But he's entirely comfortable to earn over £100k, tell his g/f he's saving for a house with her, & let her work 2 jobs to try & save up £15k. Where is his 'generosity' when you are knackered from overwork?
He tells you he is happy to put in more than me when the time comes to it - & isn't that convenient for him! Do you not get the feeling that you are being stalled, that this is NOT the 'equal venture' that you are looking for, & that he is "Future Faking" you?
He also owes one of his family members a significant amount of money
What were you telling yourself about his 'generosity' again? This man is such a tightwad he will rip off his own relative, even while he has a £25k windfall sitting in his account.
What makes you think he will treat you any differently?
he is keeping it a secret from me but happy to tell his friends
He's happy to tell his friends because they are gamblers too.
He wants to keep his nice little £25k pot to spend on his gambling habit.
If any mortgage provider knew about the gambling, his chances of approval would decrease significantly. You don't win £25k for peanuts - he is laying out large sums.
Imagine for one moment that you were proposing to buy a house with a friend in order to get on the ladder - no romantic involvement whatsoever. Supposing this friend also had a gambling addiction? Would you risk buying a house with them? Of course you wouldn't! Eventually, they will lose enough money to miss a mortgage payment here & there. Events spiral, & you end up either covering their share of the payment in order not to lose your home, or you get repossessed.
Just because you love him, it doesn;t mean that all this is not also true of him. He is a huge, ticking time bomb of a risk to you financially - all your hard work with 2 jobs & diligently saving up your £15k will have been for nothing.
I love him and share everything with him, and I thought the same of him.
Sweetheart, he cannot even share the truth with you,
He is coldly, calculatingly, lying to your face.
He is a gambling addict. You may struggle with accepting that, but if he were not addicted, he would not need to lie to you.
If he were truly committed to buying a house with you, he would be putting some of the £25k into savings with you.
He thinks nothing of leaving his relative hanging for repayment while his own cash burns a hole in his pocket.
He will think nothing of leaving you high & dry without means to pay your new mortgage should you be unwise enough to still do that with him.
Please have a long hard think about the implications & consequences OP. And don't shack up with someone so perfectly poised to destroy your future. He is a liar, an addict, a gambler, a selfish git, & I believe he is stringing you along. You will always be in second place behind his addiction. Maybe even 3rd place, behind the friends he shares news of his win with ...
I am sorry to be so harsh OP, but this is a big wake-up call for you.
I would take my savings & run ... & keep myself open to meeting a truthful, sharing, kind & non-addicted man.