Don't be disheartened! Reading wants to attract bright children with lots of interests, so it sounds like he'd be a perfect fit.
One thing to check... do you know what kind of mock he did? Reading uses FSCE (in fact, they're the driving force behind it) and they specifically designed the test to to be resistant to tutoring.
If the mock was based on a GL paper, then it may not tell you too much about how he'd perform in the FSCE test. It'd be completely different. (Reading has a familiarisation guide on their website, if you haven't seen already.)
In terms of tutoring specifically, FSCE tests a broader range of subjects than a classic 11+ (it can cover geography, music, languages, etc). So unless your son needs extra coaching in maths/English specifically, I wouldn't recommend tutoring.
His time's better spent reading widely, doing puzzles/maths problems and developing his interests. That's what Reading is looking for, rather than a child who's been drilled on solving non-verbal reasoning puzzles :)
Good luck!