Welcome Aunt Maisie. ( I do apologise in advance if other people start calling you that now)
As mentioned, I will second the advice for Corinda's 13 steps to mentalism. Though it might seem a little heavy for an 8 year old you can go through and pick and choose what pieces you can help him work on.
Other than that, might I suggest a Karl Fulves book (this man is quite famous in the magical world for writing books full of effects that work on their own, without any real work involved on the part of the performer)
The one I had in mind was his "Self working mental magic" which helpfully enough you can read a review of right here on talk magic
http://www.talkmagic.co.uk/ftopic8775.php
Time Magazines Person Of The Year 2006.