When I was at school I was somewhat of an anomaly as I came from a background where you wouldn't really expect achievement. I think it was by Y4 they didn't have any spelling tests left, so we'd play a game where you got a long word and had to make as many words you could out of it. Either Y4/Y5 it was deemed I was at GCSE level. On entering secondary education I was put forward for all these psychometric tests, they all said I'd achieve A* at exams in years to come, in fact I sat some exams early as a test and got good grades. They felt that as the primary school did, my parents should be pushing for a scholarship to a private school. As they would push me and keep me engaged and there was talk of future Oxford/Cambridge alumni.
My teachers at Primary school got the test for this really well renowned school. I came out being offered a scholarship, my parents said it would be unfair to my siblings. Oh how I hated them that they were willing to pay for everything, I think my parents said we couldn't afford the uniform, the trips, they said don't worry we'll cover it all. But the offer was turned down. Even school friends said my parents were stupid for not affording me such an opportunity. Boarding in our crazy house would have been bliss, plus I cry (not literally) when I think about the library I would have had access to.
All you can do really is hope that he passes his 11 plus. You don't need to sit him down and say ok now we're learning Pythagoras. But get some test papers and see how he does.
Does he enjoy attaining, or is he a bit of a Jack the lad? One thing I found at secondary school was you can have pretty bright kids, but they have to want to engage.
If he's getting bored and messing about, maybe advocate he takes a book to read. If you were going to get him pushed forward a year, you would have been better doing that last year maybe. Consider volunteering for the PTA as the Ofsted issues can be a management failing. So with all the will in the world nothing changes till leadership is put right.
Most importantly find out if your son wants to go to Grammar school, or if he wants to stay with his friends. But say to him that Secondary school pretty much determines your future, which I'm sure many would disagree with. But for University if that's an ambition, he's got a better chance of going better than normal from a Grammar School.
I guess it's a lot to take in when you're 11, but you have a vague idea of where you want to be.