Do you mean that, rather than now pay back what he borrowed, in cash, he wants you to take a charge on his house for it instead, so that what you have lent him becomes secured against his house ? The idea being, that if the worst came to the worst, the house would be sold and, out of the proceeds, you and the holders of the mortgage he already has would be paid off?
If that happened the debt to the existing mortgagor would be paid in full first.Your money would come out of what is left.
It shouldn't be difficult, 'on paper' or in legal terms, but you must go to a solicitor to do it, with a note of what is owed and what terms as to repayments and interest you requireThe charge will have to be registered on the title of the property..I can see practical difficulties if you want your loan back at any time or he's faling to make repayments of the loan or interest to you.The existing mortgagor could be happy with what they are getting and their mortgage may not be due to be paid off for many years from now; it may have many years to run. At best, the holder of a later mortgage who tries to foreclose,if he can, has to pay off all the earlier mortgagors first (he has to 'foreclose up' ).The solicitor will advise you of the practicalities, possibility, and consequences.The existing, earlier, mortgagor always has important rights in such matters, to protect him and his security .