If your son is happy, averagely-able or a little above average then you are correct it’s not a good idea to rock that foundation.
Have you told the pre-prep that it’s your intention for ds to sit academic 7+ selection at the most academic and prestigious schools? You are not the only parent who wants this outcome for your child so I’d ask the school what they can do to support your DC’s accelerated learning, and ask them to build a plan with you - perhaps a separate tutor at this stage would be less helpful. I’m surprised they aren’t offering a better academic outcome given the small classes and what you are presumably paying. How many kids at the pre-prep end up at the schools you’re aiming for?
I appreciate parents are busy working (I am myself) so I would definitely ask myself what does tutoring mean at age 5? It’s probably not a great idea to tack on hours of extra lessons - too tiring.
What your kid needs is a mountain of high quality 1:1 attention which is what he should be getting a pre-prep. Lots of reading, puzzles, conversation, trips out to museums, hands-on STEM/maths/logic like chess or Monopoly or baking (weights and measures and proportions and “number sense”). Maybe a piano or violin class too. If the pre prep doesn’t offer it perhaps the right answer is to change school in y1.
Having said that:
I have plenty of friends who have been using Maths tuition schemes to tutor averagely bright kids since age 3 and their kids are miles ahead in maths of the state expected level, so this is definitely worth looking into. Kids are sponges and most can learn a lot more than they are given in the state curriculum.
I can’t say if these kids would pass one of the hardest 7+ exams. Some of them make the cut at our local super-selectives but many don’t.
I think it works well if you can tap into your child wanting to learn, enjoying it, and being keen to do more. It’s hard when it’s forced, because whilst forcing kids often does mean the work gets done, if they ultimately fail the selective exam or interview,they will have gone through a miserable few years with no reward at the end.