OP, if I've understood your post correctly, your DS has sat some 11+ grammar school exams for Y7 entry and a 13+ pre-test for Y9 entry to an independent school, but you haven't applied for Y7 entry to any independent schools. It isn't clear from your pose whether your DS is currently at a state or private primary, but I'm assuming he is not at a prep that goes to Y8 (or you wouldn't be asking about prep for Y9 exams).
Some Y9 entry senior schools who pre-test in Y6 don't insist on further entrance exams (e.g. Common Entrance) in Y8 and rely on reports of continued good academic progress. However many do still make pre-test offers that are conditional on passing further exams in Y8. Children who are at prep schools are expected to sit Common Entrance; this is a 2-year syllabus across multiple suubjects, and is difficult though not impossible) to prepare for outside the prep school system. Some schools also make provision for different Y8 exams for candidates who are at state schools and therefore haven't studied the CE syllabus.
I'd suggest thinking about the following:
Does the independent school you've applied to for Y9 entry require candidates to sit CE in Y8? If so, do they have separate requirements for students coming from state schools? You may need to ask the school diredtly if this information isn't on their website or in admissions communications you've had from them already.
If the school requires CE in Y8 and has no alternative provision for state school candidates, if your DS wants to attend this independent senior school then he really should move to a prep school for Y7 and Y8 in order to prepare for CE.
If the independent school runs separate Y8 exams for state school candidates, in theory if your DS is offered a grammar school place he could start at the grammar school in Y7 and then move in Y9 - but would you (and he) really want to do that? What if he's happy and thriving at the grammar school and doesn't want to move?
There are senior schools that run their 13+ admissions process in Y7, but I'm not sure why that would be of any benefit to you - he still has to go to school somewhere in Y7 and prepare for exams.
It is a really confusing system, especially for boys where the 11+ vs 13+ pre-test vs 13+ direct is more of a dilemma because more boys' school offer a choice of entry points.