If there is absolutely no way the child will ever learn the truth, I would be tempted to let sleeping dogs lie for a long time. But if other people know, the truth has a way of coming out. If the latter is your situation, I'd ask a kid's therapist about the issue and how and when to broach it. You don't want your child to be at some sensitive point (say, age 15) and then to learn it accidentally.
The first question you should ask yourself is how will this effect you child. 2nd how long has the step dad been in the picture. If he has been there for your childs whole life or most of it, and your child knows nobody eles as a father figure then I would just leave it a lone for a few years.
Also there is a big difference in being a "father sperm donor" then there is to being a "Dad" you have to take this into account aswell. If the relationship between your son and your husban/boyfriend is great and close, I see no harm in letting it wait. I hope this helps, send me a message if you have any other question.