Most scholarships are not based on grades as they given on talent such as sport,drama and music.
He is currently on a scholarship but his school has no sixth form and a good state school seems to be looking for straight A's from their students and not looking at other talents as they want to be top of the league tables.
At sixth form level there will be few scholarships for other than academic reasons. The indies may well be trying to replace some of the kids who have left to go sixth form elsewhere - such as the state GS sixth forms - or boarding or day indies.
There is quite a lot of "shuffling" at this stage as many kids move around.
I suspect ultimately that there will be many places at indies as there is no unified application system and people can hold several offers - only deciding after GCSE where they want to go.
Your DC has to go to school for at least year 12 (I'm not sure when the requirement moves to year 13) so you need to have some backstops - have a look at other indies but bear in mind that there may be little financial help, look at sixth form colleges and perhaps have a chat with the ones who have rejected you about waiting lists etc