If he does well academically I would try RGS (may have to wait to 13+ and go to a 13+ prep in the meantime but explain he is at a state school and they may be able to do something - a long shot but worth it as RGS are keen to have state school pupils).
In this area there is a real need for a second boys' school in Guildford. If boys don't get into RGS there is no Tormead, St Cat's equivalent for them like there is for girls who don't get into or want to go to GHS.
Try Salesian College (there's another local school with the same name so here's link to the right one www.salesiancollege.com/), Churcher's, St John's Leatherhead (co-ed), Reed's, City of London Freemen's (mixed reviews), King Edward's Witley (mixed reviews), Claremont Fan Court, St Edmund's, Hindhead (less academic and only goes to 16), St George's Weybridge, Cranleigh
If academic and RGS doesn't work, I have heard of boys getting the train up to Hampton School or King's Wimbledon but it is a long journey.
There is also the option of joining a prep for the rest of the year and then Year 8. Try Aldro, Edgeborough, Barfield, Amesbury, Feltonfleet, Ripley Court, Danes Hill, Parkside