Take the time you need to do this. And yes, where you are going with your thoughts to create this letter is not a place you want your mind while you have your S.
Although first day / last day is a great tool to use while with your S, as well. Imagine you are like him. Seeing the world from the eyes of a child. How wondrous that must be for him and how can you help him create the experiences and memories that will last through his life. And to stay away from the sad parts of it, but if this were the last day you would ever be with him, would you really sit him in front of the TV while you do "dad stuff", or would you focus every moment on him, because you'll never have the opportunity again...
