In the event of your mother passing away and not providing for someone then a person who is not provided for may be able to make a claim against the estate under the Inheritance (Provision for Family and Dependent) Act 1976. You must make the application within 6 months of the date of the grant of probate.
In order to make a claim against the estate the person claiming must be able to prove that they were financially dependent to some extent on the deceased. As you son is living rent free it is conceivable that he could attempt to make a claim.
If this is wished to be avoided then your grandmother should execute either a new will or a side letter in which it is expressly stated that she does not wish for any monies to pass to him other than what is specifically directed in the Will.
The alternative is for her to start charging him market rent lodgings and keep a record of this.
In the event that he can prove financial dependency and makes a claim the Court will consider what is a "reasonable financial provision" for you and will take a number of factors in to account in determining what is fair (eg. size of the estate, other claims, resources and needs of other family members/dependants, responsibilities the deceased had to you), each case turns on it's own facts and there are no hard and fast rules to work out what you are entitled to.
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