Books recommendation for 7 yr olds

[RosieProbert]

It's my 7 yr olds birthday next week and I don't know what book to get him he likes David Walliams but we have all of the books.Any recommendations? Please reply if so

13 Storey Treehouse (can get a set quite cheaply)
Tom Gates
Roald Dahl set

How to Train your Dragon

This series of books:

https://www.searchgoodbooks.com/books/the-case-of-the-vanishing-granny-the-big-top-mysteries-1