The quicker you tell your son the truth, the better. Let's just hope that he doesn't get angry with you after having not told him the truth all this time. If you feel that you need assistance discussing this issue with him, then consult with a mental health professional or the school counselor. Waiting will only worsen this situation.
How did if go? Did you manage to tell him?
He will need support because he may go through a process of bereavement for the father he never had, plus coming to terms with never having an opportunity to get to know him for himself.
I would suggest that you're honest with him, but remember to tell your son something good about his bio father as well. There must've been something good about him for you to want to get so close to him and have a child. Having something positive (however small) can help the healing process. The phrase "you can't miss what you've never had", is not applicable in this case. Even if your son uses thus phrase, at some point in his life he will inevitably hurt over this. It's natural.
It's great that he has such a kind, loving, considerate mother.