Philip Pullmans books are superb. His dark material trilogy is a cracker.
Eva ibbotson is also very good.... You should try them!
Please don't worry about what your son is reading, just be thankful he's reading.
For many years my grandad pointed to homer every time he saw me reading anything, every single time he saw me with a book, he'd try and get me to read a more worthy tome what he enjoyed. Put me off literature for years.
Have a look at some private schools 11 plus reading list and you will see that Lord of the flies is far too advanced for him. Maybe George owrwell's animal farm, if you want to blow his mind and make him politically aware. No doubt he'll get more out of it when he's 16 but he can always read it again!
If he likes fantasy, try him on the earth sea books by ursula le guin, or the harpist in the wind trilogy by Patricia mc killip.
A rollicking tale from Eoin colfer or jk Rowling isn't going to change his personality. It hasn't changed any one else's now has it, or these books would come with a health warning.
Think about it, you're not making sense, and you're really damaging your relationship with him by trying to control his thoughts and preferences.
Truthfully, what goes on in his head is really none of your business. He'll resent your interference and controlling ways.