If you remain unmarried and follow the below advice then he will not be able to claim an interest against your property, do not:-
1. Let him pay more in rent to you than he would otherwise pay in private accommodation
2. Let him spend money on making improvements to the property which increase it's value
3. Let him pay directly to the mortgage substantial sums to pay it off (you should not let him transfer to the mortgage account directly in any event
4. Get married
Provided you follow the above he will not be able to claim against the property. If you get married and remain so for a significant period of time then the house will eventually become included in the matrimonial asset pool over which you would seek settlement.
If you have a mortgage then you should contact them and tell them he is to live there with you. They will ask that he executes their standard form deed of postponement in which he states that he shall not claim an interest in th property in priority to their charge. You will be in breach of your mortgage conditions if you permit occupation without this.
You may consider seeing a local family solicitor about drawing up a cohabitation agreement dealing with the house, any assets bought during the relationship and how they are to be dealt with upon separation. It would cost around £200+VAT.
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