Hi Dot,
Amazon do the best deal I have seen so far.
This is for 6 books.
http://www.amazon.co.uk/gp/offer-listing/07475 86675/ref=sr_1_olp_209?ie=UTF8&s=books&qid=119 7323533&sr=1-209
My 7 year old son started reading them a few weeks ago and read the first 3 with no bother, but he is struggling with the 4th as they do become much more adult in content as the series goes on. I started them at the same time and am now on book 7 (I bought it seperately) so I can appreciate that it not only becomes harder because of the language used, they become much darker in content.
I think that you will probabaly find the same with your grandson, so I would either go for the set of 6 at a good price, or even get a set of the first 3, as it will probably be a year or two before he is ready for the later books.
I would thoroughly recommend them though to stretch his reading and to get him really interested in "grown up" books.