hello I have been a lone parent since pregnancy, dd is now very nearly 7yo and does struggle with this on and off.
She is not a girly girl (ironic given my username!!) and I think if she had had one would have been a daddies girl 
Today for example she cuddled up to me and said 'I want a daddy, I just really want a daddy' its really sad and I just acknowledge that it is hard for her. One of the things I find that helps is having a booked called 'do I have a daddy?' got it on amazon and dd actually got it out today and read it.
Op your son will at some point ask questions and you have to make him feel that it is fine for him to ask. I try and be honest but it is difficult to explain to a young child why their father wouldn't want to be involved and I say that as well- that I don't understand it!
Dd has seen a photo of her dad once, she was getting very upset and said she didn't even know what he looked like so it felt like the right thing to do. She hasn't asked to see it since.
It makes me feel really sad to see her struggling with it but I remind myself that has a nice home, a loving family, gets to do lots of lovely things etc She also has some friends at school who don't see their dads so she knows she isn't the only one.
Sorry am tired and not sure this is very coherent! Main points are to be honest and matter of fact, let him ask questions (don't let your feelings stop the conversation iyswim) and remind him that he has a mummy who loves him very much!!!