The normal way of dealing with this would be for you to have a second charge over the house although mortgage lender may need to agree to it. It depends whether they have a restriction against the property preventing further borrowing.
However provided you confirm that it’s not to be repaid during the term of his mortgage lenders mortgage, they will normally give that consent. It depends whether they have the restriction or not.
That would stop your son remortgaging or selling the property without you being repaid but it will not stop the house being repossessed in the event that he doesn’t pay the mortgage.
The only way of preventing that is for you to pay the mortgage every month or to pay it off completely and he then pays you rather than the lender.
The contract between you is in these circumstances worthless because let’s face it he signed a contract with the mortgage lender to pay the mortgage and he isn’t doing that so I can’t see why a contract with you would be any more persuasive.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have