If you are serious about entering your DS for SGS then I'm afraid I would start tutoring from year 4 or you may well find all the local tutors experienced in preparing children for the 11+ are already booked up.
My son had an hour's tuition in Maths and an hour's tuition in English every week starting part way through year 4. We were on a waiting list for about six months for the English tutor and the Maths tutor was only able to offer us one free slot in her timetable which we snapped up. It's true we could have opted for less experienced tutors but my DS didn't need help with the subjects so much as experience of what the selective entry tests would require of him and plenty of practice.
It might seem insane to tutor a child who is already academically able, but so many places at the Sutton super-selectives are open to out of borough children that the competition comes from a very wide area indeed and the vast majority are tutored to the hilt. Most parents feel they have no choice but to tutor to achieve a level playing field.
For the right boy they are marvellous schools. My DS is at WCGS and is very happy, has made friends and comes home fizzing with enthusiasm about what he's been doing and learning. But he has to work in a way he never had to before when he was one of the brightest in his old class, rather than just one of many academically able boys. And there is an enormous amount of homework.
It sounds like your DS stands a very good chance of SGS and he is very lucky that you just want to find the best fit for him. Hopefully, you'll get the chance to go to the Open Days this year to get a feel for all the possible schools.
We always stressed to our DS that despite all the work and preparation, a little bit of luck was needed as well, and that plenty of very bright children didn't get into the grammar schools because the test is really just a snapshot of a child's performance on a particular day. And we always tried to sound positive about the possible alternatives - and there was much to be positive about even though we felt that, for him, they would not be such a good fit.
Good luck, OP.