Thanks for your reply.
In an ideal world, it would be best for you to proceed and purchase the property jointly with your son, and ofr which you would need to go on the Mortgage. You would need to speak to the mortgage advisor as to whether this is feasible bearing in mind your age/salary etc.
If you can procee don this basis, you can then hold a different percentage share in the property and you can then make a Will, leaving your 40% share to your other children.
If the above is not feasible, you have a slight problem with gifting your son the 40% and at the same time securing a financial interest in the property. This is because your son's Mortgage Lender will want an assurance from you (normally a letter) that the money you are lending him is a gift and if they are informed the "gift" is repayable on any Sale or you are to have a second Charge over the property, they will refuse to grant the Mortgage.
On the basis that you do proceed by way of a gift, your options for registering any type of Charge are nil. Basically, whenever a client is purchasing a property with the assistance of a Mortgage, us Solicitors have to notify the Lender if we become aware that a third party is going to have a second Charge over the property. If the Lender is notified of such a Charge, they will normally refuse to lend.
Likewise, if your Son's Solicitor gets wind of any sort of agreement he has to repay you a certain amount of money sometime in the future/upon any subsequent Sale, he is under a duty to notify the Mortgage Lender.
The best you can therefore hope for, is that a Declaration of Trust is drawn up (possibly after completion of the purchase) by an independent Solicitor, which document can stipulate that the deposit is repayable on any Sale.The document is signed by you and your son. You could then instruct your Solicitor to register a Restriction on the Deeds, meaning that before any Sale can be completed, your son would have to notify you (not that he would, but this will prevent him from selling without your knowledge). This would then give you some protection- and you could then make a WIll, leaving this interest to your other children.
I hope this helps and sets out the legal position.
If so, I would be grateful if you could rate my answer.