You can get him to leave you the money in his will or claim on his estate as a creditor and/or you can get a personal bond/contract/IOU signed but my advice is that a charge on his house for "all sums due and to become due" is the best way of dealing with this. It can be done for a few hundred pounds and will rank you as a secured creditor rather than an ordinary creditor. So much as you are averse to the concept, my advice is that that is the safest way to proceed. Any other way carries risk if he has other creditors, changes his will, runs out of money or even remortgages to another lender.
If you are executor and have proof of monies paid to him that is sufficient to enable you to put the monies paid down as a debt in the estate inventory. However I still rcommend a charge. It only has to be done once as it will be for all sums due and to become due. You don't have to repeat the charge for every payment you make to him. The first charge, if drafted correctly, will cover all future payments.