If he loved you, and wanted to build a future with you, he would make sure he was equipped to do this.
As it is, he has been happy to do nothing for years, not taken any kind of responsibility for himself, coasted on his parents, and now he wants to sponge of you.
They do say love is blind, and in your case, I think it is particularly true. Who in their right minds would be off flat hunting with a 25 year old grown up man, who has never worked in his life, is going straight from his mums care to another woman who will care for him, lives on money donated to him by relatives. It is insane. They must be so happy you came along to take him off their hands. The moment he moves in with you, YOU will be responsible for him, and they are well rid an adult draining their resources.
Not because you really WILL BE responsible for him, it is just that your sense of duty and care, will force you to take responsibility. Because if you dont, YOU will get into arrears on your council tax, your utility bills, your rent, and you may find yourself homeless, and with a bad credit rating before you know it.
But, you are also a grown up. At 25 you are not young. You are an adult, and you are also living at home, so you have also no clue about adult responsibilities. Because if you had, you would have ran a mile from this man already.
I actually feel really sorry for you. So loved up, immature and naive.
If you cant even have a grown up conversation with him about money, and how he is to PAY for living away from his parents, how can he be ready for this?
In fact, if you cant talk to him about it, I would go to his parents and ask for an allowance for him. Tell them, "look, I am taking your son of your hands. He is not able to work, feed and clothe himself, let alone house himself. I cannot afford this on my low salary on my own, and I cannot be responsible for him. I need a minimum guarantee from you, his mum and dad, that at least RENT will be paid for him every month".
In fact, in this case, I think it is the most sensible thing to do. It will cause a stir, but it will highlight some very important issues.