Do I have to wait a full separation period of 12months instead of 6months if my husbands name is not on my sons birth certificate?

That's an interesting question.  Ordinarily, even if the child was born prior to the marriage the act of marrying "legitimizes" the birth and it is considered a child of the marriage.  I think if you were saying that the child is NOT your husband's child then it would be different, but it seems you are acknowledgin that it is your husband's child, even if no paternity test has been done.  It's something you could try to argue to the judge, but is it worth it?  You could go through the time and expense of filing for divorce only to have your case dismissed and have to wait another six months if the judge doesn't buy your position.  I'm honestly not sure if anyone has tried to argue this before.  In any event, you would need a written property settlement agreement that resolves any issues beteween you in order to file a diovrce after a six month separation.  Have you and your husband resolved all issues?  If not, then this question is moot.  I'm leaning toward thinking you have a valid argument, but I would still recommend waiting until you have a one year separation rather than risk it.