I would be wary about Whitgift and Trinity at 13+ as they have very few places at that stage. Dulwich is different as they have a proper intake at 13. The number of places available also changes year on year. Whitgift only had 5 places this year as they had too many 11+ acceptances two years ago. The Whitgift headmaster does not like 13+ entry.
You should also consider that he will not be able to pick up a new language at Trinity at 13+ - he can only continue with the languages he has studied up to that point. Whitgift will only allow Italian or Greek as a new language at that stage.
When you say RR's results are worse than the Comps which schools are you comparing it with? Are they Comps that you actually could get a place at or are you looking at places like Coloma which have amazing results.
RR has a reputation for being good with children who have various difficulties and my impression was that it is a lovely, welcoming place - it also has girls which could be a positive for your ds in years to come!!
I went to visit St Dunstans and, frankly, I hated it. It is right on the South Circular and struck me as cramped and grim. All of the children I saw were untidy and quite loud. The sofas in the reception area were torn and the floor was dirty. I thought that if they couldn't even be bothered to get that bit right than what does that say about the school in general?
If your ds enjoys his prep school then it may be the best place for him but do bear in mind the caveats above. Good luck!