My Y6 DD has just got into the right level to get the Gram of her choice.
My DS is in year 4 but is in a year 5 class. School have not said anything as yet but I am worried if they suddenly say they think he should go straight on to y6 when I will not have done his 11+ prep. Hopefully not.
I did 11+ prep with DD from Spring term in year 5.
My ds was in the same shoe like yours but he was moved to year six due to our request for him to do the 11plus as he was just about six weeks out of the school chronological age and he is been working in Year six since he was in year 4.
I prepared hin for 11 plus in kent and Bexley with some of his friends and they all passed but he passed with 420 (100) in Kent and 280 (100%) in Bexley.
So if you want him to go a year earlier like some ind schools pupils, you can start preparing him now as I now another girl that was in Grammar school a year earlier and some Grammar schls in Kent and Bexley do accept them.
I will agree with Yoyo that start working on his vocabulary and Mental maths, time table and KS2 maths . Some of them have big brain that can cope with some of KS3 maths.
Wish you the best in search of acre of diamond in Kent