There's not really that much choice TBQH, unless your DS could get a place at one of the neighbouring Wandworth comprehensives, or is bright enough to do the Sutton/Kingston 11+ exams.
There's Ernest Bevin (boys),
Graveney with a very good reputation and selective stream (but it means doing very well in the Wandsworth Test otherwise the catchment for general places is about 700m)
Chestnut Grove (co-ed) has specialist places for those with aptitude for art and languages
Ashcroft Academy (mixed) has specialist places for computer science and something else I think
Coombe Boys (just into KIngston) is very popular but unless you're in Raynes Park/West Wimbledon you're probably too far away to be in catchment.
The new Harris Academy Wimbledon (co-ed) could be an option. First Year 7 cohort entry will be in September 2018.
Other than that there are the two other Harris Academies Merton and Morden to consider. And St Mark's CofE Academy (also in Mitcham).
Also just about to open is the new Harris Academy Sutton which is going to be very STEM oriented and have close links with the proposed London Cancer Hub. As it will be in its early days, your DS might get in?
Wimbledon College is an option if you're Catholic.
Think that's exhausted the options I can think of. There are good comprehensives in Sutton too (Glenthorne and Greenshaw particularly).
Glenthorne offers 25 places every year to youngsters talented in drama, dance and/or music (and you appear to stand more chance of getting a place if you are good at all three!).
Greenshaw has a selective stream but it's quite competitive with many of the children who miss out on grammar school places choosing to go there (even if they live out of borough).
There is always talk of choice of secondary schools but there isn't really that much, unless one's child is academically bright, gifted in another discipline, has a faith or can consider private education as an alternative.
Happy to answer any more questions if you have any.