Reading the same book over and over again

At the moment, Harry is obsessed with a particular series of books. It's the Percy Jackson books by Rick Riordan, which he was introduced to at school. From what I understand it's about a teenage boy who is a Demigod, and he has many adventures with his friends which involve various Gods, good and evil. We've bought him several of the sets - initially I baulked at the price for a whole series but they have turned out to be an excellent investment as he has quite literally read them all over and over again. When he reaches the end of a series he starts back again at the beginning.

I must admit that I get a bit frustrated seeing him constantly re-reading the same books. I take him to the library to try and tempt him with books by other authors, and I've researched authors with similar styles. As it's a popular series there are plenty of other books that are in the same genre and cover the same subject matter, and he's rejected all the alternatives that I've offered. I feel it's a shame because he's such a confident reader and I want him to broaden his horizons a bit.

I've tried to read the books myself to see why he likes them so much, but I must admit that I can't see the appeal myself, I guess because I'm not in any way the target audience! I know that he's interested in the stories of the Greek myths that the stories base themselves around, and I think a lot of it is that he's 'comfort reading', using them to relax and escape at the end of a busy day surrounded by people.

But then I remember that I used to read the same books over and over again when I was younger. For me it was mainly Enid Blyton, starting off with the younger Famous Five stories and moving on to Mallory Towers, and I can still vividly remember many of the books. I have many books on my bookshelf that I've read numerous times and could easily pick up now and re-read without being bored, and many books that when I re-read them they take me back to all the times in my life when I previously enjoyed the book.

So maybe it's not a bad thing and he'll move on from them at some point, I just wish that he'd at least entertain the idea of reading a book by a different author!