well I adopted my son as a single mother and his birth mother disclosed absolutely no information about his birth father at all - you don;t get much more dad-less than that.
He is four and asks me why he doesn;t have a daddy - I explain that like everyone he had one when he was born but we don;t know who he was. You might like to use the line I use about Birth mother "some people can only look after themselves and babies need looking after immediately so you came to live with me becasue I needed a son and I could look after a baby". In DS;s case I say that there are all types of Daddies and some are excellent but some are not very interested in being Daddies and we didn;t want one of those so perhaps (!) we will find another Daddy one day who is good enough for us. UNtil then X and X and X (insert as many names as you can think of - older children aunts uncles anyone you can dregde up) also don;t have a Daddy living at their house.
I don;t know how old ypur DS is but my DS's school have said the single parents become more and more common as school goes on.
I also say "no we don;t have a Daddy in this family but we do have a Nanny who we love and a lovely UNcle Ian and etc etc etc and we are so lucky as many people don;t have that"
Occasionally I also ask him "does it make you sad that we don;t have a Daddy living here?"and ask him why - often its something practical and can be sorted with an appearance by my ever reliable brother!
If he is older worthwhile looking ion the net for famous people, sportsmen etc who were raised by single mothers.
ALso acknowledging (depending again on age) that its crap to have a fatehr who isn't good enough for you but the compensation is that you get loads more of mummies time than if a daddy lived here.
DS is all in favour of moving a daddy in but making him have his own bed so that there is room for DS in mine!
Good luck