My research suggests that in NJ, although a writing was not executed in compliance with state law regarding Wills, it will be is treated as if it had been so executed if it can be demonstrated that the writing establishes by clear and convincing evidence the decedent intended the writing to constitute an addition/alteration of their existing Will. So this letter may stand (or it may not). That having been said, to be sure that your grandfather's wished are carried out, he should just make a new Will. Forms are available online and are inexpensive. This would be a better way to handle this situation. Or you could consult with a probate attorney to get their adise based on specific state law.