Thing is, he is a consistent level 5b at the end of Y5 in English, his weakest area. I highly doubt his IQ HAS gone down, his knowledge of Mathematical Concepts and the depth and breadth of his Scientific knowledge attests to his IQ. It just so happens that English Comprehension is his weakest subject. That and Drama.
He is also very good at languages, and wants to learn Latin to help him pursue his dream of a career in Medicine.
The Grammar school teaches Latin from Y7. There is NO opportunity to learn Latin at the closest Secondary school, which he would be certain to be allocated if he doesn't pass the 11+ and get offered a place at the Grammar school, due both to proximity and a sibling link via his older sister.
He has thought this out long before now. And as for the school 'tutor', he is a supply teacher the school keep on their books, who is used to give tuition to DC's, usually to help them catch up on work that hasn't been taught if they have had one of a couple of particular teachers.
IMO, having two of these teachers in a row is why DS1 is having this tuition, along with the rest of his class, so as the school can still be seen to be making the 'appropriate' levels of progress between KS1 & KS2 SATS.
It is telling that the class that had one of these teachers is also getting the tuition this year. And the other one has left the school under mysterious circumstances, with no further job announced on the newsletter (never happened before...)
I think that the tutor is actually hired for this reason, but given DS1's achievements in most areas, the school directed some if the tuition time towards the 11+, as they make a big hoo-ha each year of x amount getting into the girls Grammar and y amount getting into the Boys Grammar, conveniently ignoring the fact that with a relatively affluent intake, the DC's will have been extensively tutored outside of school.
And if DS1 DOES have gaps in his knowledge, IMO it has nothing to do with his IQ, and everything to do with the fact that he had poor teachers for Y3 AND Y4.
I make up what I can at home. But I'm not a teacher or a tutor.
And if the tutor had been giving me a realistic idea of his progress, rather than telling me he got 98%, 99%, 100% on tests, and told me that he DID need more work on Comprehension, then I would have been doing more at home with him much sooner.
I am working on Punctuation with my 8yo with SEN, as it was highlighted in his school report. Mostly because he has never been taught it, as he was being taught in a 'remedial' group until very recently, and he has been expected to go from the 'remedial' group back into the classroom, while somehow apparently being meant to possess skills he has never been taught. 
I have no problem working on things at home with them, but what I was getting from the class teacher, (also the Deputy HT for Y5) the HT, and the tutor at the school was that if he didn't pass the 11+ and get into the Grammar they would eat their hats...
They all have told me that he is the brightest boy they have ever seen, and is truly gifted (so they keep telling DS1 and me). They have not ONCE told me that he has issues with his Comprehension that would possibly cause him to miss out on a place at the Grammar.
And if I had been paying for the tutor he has now, months ago, (not that I'm paying now, mind you), these issues would have been picked up on months ago.
So it is purely down to lack of money, IMO, why DS1 may miss out on a place at the Grammar school. Which saddens me, because I truly believed that it was the best way out of this for my DS1.
And I have spent the last god knows how many years defending Grammar schools against accusations of 'selection by the back door' of those DC's whose parents had enough money to tutor them to the test. I truly believed that the 11+ would be a test that was able to be passed on academic ability alone, irrespective of tutoring and hot housing etc.
It remains to be seen how DS1 will do in the 11+, but my hopes are slim now. And for a DC as bright as he IS, limiting his options by him ending up at a school that may not offer triple science, and definitely doesn't offer Latin, is doubly frustrating.
FGS, this is a child who can explain Quadratic equations at 10yo, and draw some graph thing from them that I don't fully understand as an adult. This is a child that can have an intelligent conversation about molecules, and ionic and covalent bonding of atoms. Yet because he hasn't been taught to interpret various pieces of text and gain answers that are inferred, he is undeserving of a place in Grammar school?!