@rats82 here is a run down of all the local schools in Dulwich:
Dulwich Prep London is a separate entity with no affiliation to Dulwich College and prepares for the 11+ and 13+. Girls are only in the preschool and after that its boys only. It sends boys to the likes of Eton, Westminster, Whitgift and less selective schools like Royal Russel etc. However, the majority go to Dulwich College for senior school. Its a great option if you aren't sure yet how able your son is and it keeps options open. Its academically selective but as they take children from 3 the intake is naturally a bit broader.
DUCKs is a co-ed nursery and pre-prep affiliated with Dulwich College. There are multiple entry points (baby, 2+, 3+, 4+) and it prepares children for the 7+ examinations for Dulwich College Junior School and the other local independent schools like Alleyns, JAGs, Rosemead etc. The majority of the boys have always gotten places in Dulwich College Junior school at 7+ however, it's not been automatic but dependent on performance in the exam. This may have changed recently though so its worth ringing the school to confirm. Even though DUCKs prepares kids for the 7+, Herne Hill has a better reputation locally for getting more academically out of its kids for the 7+ exams.
Your other options in the area are all co-ed. Herne Hill as previously mentioned is a stand alone pre-prep that is geared to the 7+ and has a very strong track record sending children to JAGs, Alleyns and Dulwich College. Herne Hill has official entry points at 2, 3 and 4 years old.
Rosemead and Oakfield both are co-ed prep schools in Dulwich that prepare for the 11+ and have attached nurseries for children from age 2 as well and multiple entry points (3, 4 and 7). These schools are more mixed ability in their intake but Rosmead in particular regularly sends a good portion of its students to Alleyns, JAGs and Dulwich College with scholarships for art, sport and music.
Not sure if you are planning to have more children but if you do have a girl, JAGS is another top through school with admission at 4 and 7 as well as 11 and 16. Sydenham High has a broader intake but is a very popular all girls option within easy commute of Dulwich with similar entry points to JAGs.
No matter what the ability is of your child, living in Dulwich you'll have a wealth of choice including the outstanding state schools. Dulwich Hamlet, Dulwich Village Infants, Judith Kerr and the Charter North Dulwich for secondary. The latter two schools are amongst the top performing state schools in the country (top 1-2%) with huge value-add scores.