I don't see this as a police matter.
Does your ds have a clear story about his father that he can use?
So maybe, " Of course I have a father everyone does. Mine split up with my mum when I was a baby and he doesn't live around here anymore"
Or, " of course I have a father everyone does but I don't see mine because he wasn't kind to mum and me"
Ideally he can finish the statement with "but I hang with xxx instead so it's ok"
Xxx being Grandparent, uncle or whoever male role model is.
Or some variation of what situation is.
Your son is at the age where dc tend to focus on same sex parents so it isn't surprising he is thinking about this.
Having a clear narrative about his situation should help him.
The more confident he is in the situation the more able he will be show others he really isn't fussed and the less grief he will get.
Talking to school might be a good idea as they might have good pointers. I would be very surprised if he was the only dc raised by a lone parent.