DS hasn't seen his dad since he was 9 months. He is two n a half now. When should I expect DS to start asking about his dad? ATM he has no idea he is supposed to have one. No memory or anything of him.
As soon as he becomes aware of other families, I would say - so depends on friends/nursery/books/TV. My dd has never met her dad, and asked about him from pretty early on. She always asks more when we have been visiting/holidaying with 2 parents families with kids her age (which we do regularly - my friends/cousins) and she has been observing the different dynamics. But like toys, pets, anything, when he becomes aware of other children talking about theirs, he will want to know why he doesn't have one too!
See DS has seen toddler groups and it is always mums there. He has seen my bro's parents in law and heard my SIL call her dad dad and has since started calling me daddy or mummy. Not sure if that means he just thinks I am his whole package or if he is confused about what a daddy and a mummy is. He gets a lot of his male needs from my father and totally adores him.
My DD didn't ask about her dad until she was 3. I told her that some children just don't have dads and she accepted that at the time and carried on with what she was doing. She still ask about him occasionally, "where's my daddy?" what else can I say but "I don't know". Children are very resilient though and if it's the norm then they accept it. You might find more questions arise when they start school and realise that some of their friends have 2 adults in the house, not just one.