Pruni - don't worry abut yoyr DS - he will be fine fine fine. What you need to do fo now is look after you. This is about YOU as a Mum, not about your Ds as an only or a sibling. Children with no siblings grow up happy, children wih heaps of siblings grow up happy. Mums and dads who don't feel they have done all the mummying and daddying they would like to do might take a while, and have to try several tacks, before they can feel ok about that.
I'm very sorry you feel so fragile at the moment, and that you are not where you wnat to be.
But try not to project it onto your DS. Of course you don't say 'sorry' - you say 'this is our family you, me, and daddy'.
he needs to know that he is enough...that you are not destroyed because another child was needed. That will be very very hard for you at the momen, but you don't need to be doing much that is pro-active - just don't apologise or make it seem to him that the family has a missing place fo a child, that can't be filled by him - iyswim.
But you know - he will be fine. I know that , I am the mother of a content 7 year-old 'enfant unique'. Spend some time on you.
And no - one is going to mock you - that's no what that was about.