If you purchase the house in both names make sure it is as tennants is common and his share is only the amount he has put in ie 5%. However this could still be risky if house prices increase drastically, and things do not go well between you he would be able to claim his percentage of the total value.
A better way may be for him to register a second charge on the property for the amount he is lending you. This can then be discharged when you have paid him back.



