One other thing I would bear in mind if you're considering all three schools is that their entrance exams are quite different, so that's going to take careful planning.
Tiffin has multiple-choice English and Maths at the first stage, followed by a second stage involving written English and Maths, including longer written responses.
Hampton is particularly different from a standard 11+. Its English paper includes comprehension and creative/descriptive writing, so being able to produce a good piece of writing under timed conditions is important. It doesnt look like this was included in his assessments. There are also separate reasoning and maths papers.
RGS also has an online multiple-choice assessment element, alongside a written English/creative writing component.
So if he is going to be prepared for these schools, I'd make sure Year 5 preparation covers:
Comprehension
vocabulary
reasoning
spelling
grammar
punctuation
regular timed creative/extended writing.
I wouldn't rely solely on GL-style multiple-choice papers or cgp books.
Given his scores, I think he has a very promising starting point.
However, I would want to work out what is behind the lower 115 reading score and build that area during Year 5.