Make it an interest-free loan but state that the size of the loan increases with the value of the house. Then when you die, the value of the loan will be added to the value of the estate and and he receives his share of the estate, it will be inclusive of the then value of the loan.
Assuming your estate is to be divided equally between your children. It puts them in an equal inheritance position
Example:
You lend him 25% of property say £60,000 on a £240,000 property
When you die, your estate is worth £500,000 and his property is worth £400,000 so the loan has now increased in value to £100,000.
Your estate is worth £500,000 +£100,000 + £600,000.
Each child inherits £300,000. Your son's share is made up of the £100,000 invested in his home plus £200,000 from other assets and your daughter gets an equal amount.
We have done something similar with one of our children.