OP, it's sounds like your son is in a prep that prepares them for CE at 13+?
Or are you thinking of moving him at 11 plus (ie going into Year 7)?
Surely the school are advising you?
Look at the Best Schools to get an idea of rankings by exam results..
But don't get to hung up on 'the best school' because a) admission is fierce and b) the differences in exam results that determine those rankings are minuscule (eg a decimal percentage figure between the 'top' school and others in say, the top 20). Eg. If Westminster gets 98% A and a school like Highgate gets 95%, it's hardly a great shakes is it? Even if a school achieved 80% A, as long as your child's grades are in the upper 4/5ths of that cohort, it's the same difference.
If you are considering 11 plus entry and are prepared to move house, you could look at the 11 plus 'super selectives' such as Highgate (co-ed), Latymer Upper School (co-ed), City of London Boys or Hampton School. Bear in mind though that these are super- competitive entry with around 10 applicants per place.
For 13+ entry, don't they tell you what his scores are from a whole series of mock pre-tests and then advise you accordingly? Different schools will be asking for different scores in pre-tests.
What are the current school saying?
As for university destinations, there is no such thing as a any school "getting them in" to Oxbridge or whatever. All uni applications are strongly contextualised.
For 11 plus or 13 plus entry, another 'top' school is Kings College in Wimbledon.
Basically, off the top of my head, the top (academic) schools in London for boys (inc co-ed options) are -
Westminster
St Paul's
Kings Wimbledon
Latymer Upper
CLBS
Highgate
Hampton
Haberdashers
Merchant Taylor's
Those are probably the 'top tier' and an able child will probably do equally well in any of them. At that level, it's more about the atmosphere in the school and whether you like it. Also, whether you want a co-ed sixth form because some boys schools become co-ed at this age (ie. Kings and Westminster).
Also, Kings Wimbledon offer the IB as well as A-levels if that's of interest.