And, I would gently ask, are you happy that he is getting something out of the school he is currently in? Excellent pastoral support or sport or drama - something that he is good at? Because all kids have their strength and I assume/hope the school you have paid for him to be at from 3 have found his strength and are nuturing that talent?
Most prep schools are linked with a secondary, and the one thing they do is prepare their kids for transition to that secondary - if they have had your son from age 3 they have had plenty of time to help prepare him - and, if they still think he is not going to reach the standard, I would expect, that one of the things you have been paying for all these years, is their expertise in suggesting suitable alternatives for his particular strengths and talents.
But please also remember..."Has anyone else had this problem, what happened, did they go to state school ??"....this does come across a little "oh, god forbid, not a state school... as if it is some horrid disease!!! Possibly they did go...the vast majority of the British population do, after all, go to state school and the vast majority do, after all, come out and make their way very successfully in the world!!
As Loobylou et al say - if you give your location in the country, and maybe the sorts of things your son loves (drama, art etc) plenty of people will be able to suggest alternatives.