Your thread title tells me there’s a lot of confusion and misunderstandings here. The Sixth Form Scholarship and the King’s Scholarship at Eton are two completely different types of scholarships. There are a plethora of scholarships at Eton, some conferred honorary with no financial values attached to them and some, though limited to 10% of the fees officially, but in reality can be worth up to more than 100% in combination with a (massive) bursary that almost always goes together.
The King’s Scholarship is without any shadow of a doubt the most prestigious of them all and is firmly enshrined in Eton’s foundation. Some may even say it’s the foundation itself i.e. the King’s Scholars. Boys take the KS exams at the age of 13 and successful boys keep the scholarship for 5 years i.e. his entire career at Eton. It is said, for example, not even the Head Master can dismiss/expel a King’s Scholar who are/were literally the King’s scholars and not the Head Master’s who himself is just an employee of the King. In the modern world, The Provost (who represents the King) works very closely with the Head Master who in reality has much influence on The Provost.
The modern world also saw the introduction of many other scholarships at Eton for excellence in academic work, music, drama etc. Some are more specific e.g. only for boys of Arab/Palestinian origin or of Russian heritage etc. The 2-year Sixth Form Scholarship is designed mainly for boys from the state sector though boys from the private sector with a substantial bursary is also eligible to apply.
The KS is now out of the question for your son, OP. Your son’s predicted GCSE grades are very respectable. You cannot compare them with boys from Eton of course, who would have had at least two or three years solid Eton education prior to doing their GCSE’s, not to mention the many years spent at premier prep schools in most cases by these same boys. The Sixth Form Scholarship is designed specifically for boys like your son. Needless to say, it is a very competitive scholarship (like all other Eton scholarships) and having great and respectable GCSE grades is one thing but Eton is looking for more than just great academic achievements. A great all-rounder IMO stands just as good a chance as a solitary academic, if not better. Go for it and do your best.