MGJO We came back from an International School where they had only actually heard of one Girls' London Day School! But actually having done the 11 year entrance exams there with DD1 and here with DD2 I think it is a huge advantage. DD1 had some minimal tutoring out there, to bring her Maths up to end of Year 6 level because they hadn't covered everything yet and some help with English which isn't her strong point (turned out she is actually dyslexic ) and she got in everywhere. Certainly the very selective girls' schools are looking for potential and very much value the applicants having lived somewhere else. At her International School (on a level and with the ethos of a good state school) she sat on the top table of 5 but certainly wasn't the brightest in everything. Presumably you have some idea where your son sits in his class in terms of ability. And in spite of not being crammed she had absolutely no problems once she started.
Going through the process with DD2 back here was horrendous mainly as a result of parental panic and pressure. In fact so quickly was I recovering my daughter at the school gate so I didn't have to engage with it that I could have gone straight out with the SAS on a hostage recovery mission. The school, under pressure by the parents, became a cramming machine, and on top of that parents were engaging tutors left, right and centre. In the end it made no difference, in fact may have caused some to overheat and underperform. The bright ones got where they were meant to go regardless.
I would relax, let your son find his level, then in Year 5 come back, visit the schools, see which you and he feel are right for him. DD1 chose where she wanted to go, not the one the school had heard of, and the Head there got the name wrong when she announced it at her graduation! Talk to the schools about what they are looking for and want him to have covered, IME you will hear all sorts of hysteria from parents and the schools will be anxious to balance it out. Most of the schools will let you sit the exams in their overseas school.
For what it is worth I really like KCS and Hampton boys, seem very rounded and grounded and always happy at school, and the bright ones do very well. (Don't forget the schools with the best results get them partly as a result of selecting pupils who are good at passing exams in the first place, it doesn't necessarily mean your child will do better there, in fact may do worse if they lose confidence in such a bright cohort). St Pauls will suit the outstanding all rounder with lots of confidence but beware they may get a even fuller of themselves. Westminster is great if they have a quirk and a spark, it's a very stimulating environment but might be hard to keep up if they didn't have some sort of quirk to make them interesting. Eton is what it says on the tin, not necessarily for the brightest but will deliver you an all round patrician.