Your nephew is rather young at eight years of age to be thinking of taking the 11 plus unless the school have told his parents that he is ready to take it. If they haven't, I personally would not embark on teaching in the conventional sense or engage a private tutor at the age he is.
Personally I would, through good communication with him and having fun exploring, introduce a variety of subjects for him to acquire an enquiring mind. If he has an active and enquiring mind he will naturally ask many questions which he and you will be motivated to find the answers to if he is sufficiently interested. He will then more readily retain the knowledge he has discovered, I believe.
Albeit we all learn from books, I believe it is equally important to develop the ability to analyse, observe, problem solve, improvize, question in a constructive way and debate the pros and cons of. These we can do every day by noticing what is happening in general.
A free, uncluttered mind is more receptive to learning than one that has unnecessary strictures placed on it.