Obviously I am so not with the times ... as I have never heard of this being done.
It's a great idea. Though, a tough one for anyone as to what to write.
And how much? how little?
I think the really hard part would be to determine what is important to say now,
and yet be relevant 18 yrs from now too.
Like the others, I think I would suggest writing about your friendship with his Grandmother. How it started, and how it endured. Special times and not just the celebratory ones. Perhaps something his Grandmother did for you, when you were struggling in life, one way or the other, for him to gain a better insight into her personality.
Like
RedStilettos, I thought of telling about your own 18th birthday, your life at the time and what was happening in the world then. So much can be gathered from the internet.
Another thought might be to write about the things you know now, that you wish you would have known at 18 and how that might have changed your life.
I agree with you ... somewhat a tough challenge when you probably will never meet this young "man"!