Your DS is not saying he wants a sibling because he really wants a tired pregnant mum who can do less fun things with him, followed by a mum who has much less time and energy for him due to taking care of a newborn who is stealing his thunder and your exclusive attention.
He envisages a sibling to be someone roughly his own age to have adventures with.
That's not to say children don't come to like having siblings but it really won't break his heart if you say "every family is different and ours is special with just you and me".
As he gets older, he will learn how babies are made and realise why he doesn't have a sibling.
I think it's important not to project your feelings on to your DS. You may well be heart broken about not having another baby but that is your feeling, not his.
To accept and move on, you need to recognise your own feelings and desires - perhaps those desires cannot be met, sadly that's life sometimes.
Sounds to me like you have an enviable set up - a gorgeous son, an amicable co-parenting relationship with his father, time for yourself and time for your son plus comfortable financially.
Pretty ideal really.
Would you really want to put that at risk?