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The only way of giving you 100% security in the property for the rest of your lives would be by the grant to you of a "Lifetime Lease" by your Son.
However, if he proposes to take out a Mortgage on the property, his Lender will not agree to such a Lease, as this will affect the value/saleability of the property, in the unfortunate event of your Son not paying the Mortgage and the Lender repossessing the property.
Indeed, if your Son is going to have a Mortgage and you will be occupying th eproperty, the Lender will require you to sign a Form stating, that any rights you have in the proeprty are postponed behind the rights of them as Mortgage Lender (ie if they did repossess the property, you could not then turn round to them and say that you have a right to remain living in the property).
I am afraid, therefore, that you are not going to be able to obtain any security, if matters proceed.
The only thing you could possibly do is to become joint owner with your Son and this would also mean you both going on the Mortgage. If this is indeed feasible, it would mean that you have a right to remain living in the property as one of the legal owners. I am not sure, however, whether the Mortgage Lender would be happy to grant you a Mortgage as I do not know your personal circumstances/age etc.
I am sorry this is not the answer you were looking for, but it sets out the legal position.