I'm not of much help here BUT wanted add a little story ... When my nephew was about the same age i didn't know how to towel dry him down there after his bath ... interesting enough he told me i wasn't doing it like his Mum and dried himself ... i was amazed at how open he was about it ... and if anything i felt embarrassed that i didn't know how to go about it properly when i am the adult ... but he clearly knew how to look after himself there (obviously with the previous guidelines of his parents help) ...

I think Alan is so right in that your DS needs to explore and in hindsight then learn from you and/or DH at some point in how to clean, etc ...