I would think very, very carefully about what you?re doing before you go ahead and do anything.
Firstly, does your dh know that your ds might not be his? And more to the point, does your friend know that your ds might be his?
If you get a paternity test, you won?t be able to do it on the quiet, samples of either your dh or your friend will have to be taken to prove or disprove paternity, and at that point everybody involved will need to be told. If the baby is the other man?s, how do you think your dh will react? How do you think your friend will react? If the baby is his he will be entitled to access and may demand it in fact, are you prepared for that? Are you prepared for the fact your dh might leave if the baby is proven not to be his?
While I agree to an extent that the child has a right to know who his biological father is, I think that there?s a lot more to it than just that, families might be ruined by this, if your husband is a good father, and you are making a go of your relationship, then I think there is potentially nothing to be gained by having a paternity test and everything to be lost. After all, it only takes on shag to father a child, but it takes a lifetime to be a father.