Hi Trants
I'd say you need to be getting on the case now. Are you already tutoring your DC (or does he have an external one)? Don't want to panic you but it's only about five-six months until the initial 11+ exams.
Have you visited the SW London super-selectives yet? Most have their 'open evenings' this side of the summer hols so you need to check out the schools' websites for diary dates.
Also, you will need to complete SET application forms (one that covers the Sutton ones and another for Tiffin) for your DS to be eligible to sit the 11+ exams. From the SGS website I've just found this info:
"The Selective Eligibility Test (SET) 2017, for entry in September 2018 will take place on Tuesday 19 September 2017. Registration for the SET opens on 2 May 2017 and closes on 18 August 2017. The online registration form will be available on our website, and on the websites of the other participating schools.
The SET is the first stage test common to Greenshaw High School (selective band), Nonsuch High School for Girls, Sutton Grammar School, Wallington County Grammar School, Wallington High School for Girls, and Wilson's School."
Note that it's better to do the registration 'as soon as' rather than waiting until summer hols (when it's easier to overlook such things).
It's not necessary to do the Mock Tests BTW - although there is a lot of SW London panic chatter about the necessity of doing so. I guess it does show you where your DS sits in the cohort , highlights strengths/weaknesses and gives practice in sitting a formal exam (which can be quite intimidating/daunting for an average 9/10 year old).
Competition is fierce and getting more so by the year IMHO. All the DC I know of who've got in in the last year or so have been tutored for a matter of years (rather than the few months of home 'DIY' tutoring with parents that used to be more normal).
Good luck!