welcome to the forum!
He does read books like Roald Dahl and Harry Potter, but does not seem to learn much vocabulary though.
Here is a reading suggestion list which could help your child to diversify the authors:https://www.elevenplusexams.co.uk/advic ... ading-list
The thread on exams contents can help you to see the level of vocabulary the children are expected to be familiar with:viewtopic.php?f=11&t=14423
It is usually considered useful to read some classics besides the more modern authors...
Enjoy the prep; it is an occasion to share many moments with your DC