Yes, though it may not be the best option. Get him to sign a charge in your favour, register it and, when you pay off the other mortgage, you will have a claim on the property which gives you the right to possess the property in the event that he does not repay you. Clearly, your agreement is that he does not so you have protected your position and will be able to acquire title. If he continues to live in the propery, you should formalise the arrangement. Go to see a solicitor as, if you get it wrong and he dies or absconds during this process, you may be out on a limb. .