What is the derivative beneficiary?

Was your father married to your mother at the time of your birth or ever? If not, did he legitimize or accept you as his child before you turned 18?

To answer this question, we need to know how your father immigrated. A derivative beneficiary is normally a minor child or spouse of a beneficiary of a family or labor petition.

Although it is a good idea to have an attorney carefully examine all relevant documents before a conclusion be made, I am pretty confident that whatever was done cannot be undone and therefore you could be excluded from any free ride to which you could be entitled then.

Since you are married, you are no longer a derivative beneficiary.

The answer is that a derivative beneficiary is generally a derivative beneficiary by operation of law, not by choice of the principle applicant. So even if your father had his "second" family" on the paperwork, you would still be considered a derivative beneficiary of the visa petition filed on behalf of your father if you otherwise meet the requirements for such classification. I am not sure how this fits within the scope of the inquiry that you have made, but having derivative beneficiary status may actually qualify you for the benefits of 245(i) of the Immigration and Nationality Act to the extent that this helps you in any way. My suggestion is that you sit and discuss your situation with a licensed and competent attorney. Rather than make guesses as to whether you are or are not qualified for any immigration benefits, someone with knowledge and experience in this area should be able to explain your situation and provide you some guidance on how you can proceed in legalizing your own status.

No, if you were not claimed at the time, you might still qualify for citizenship (you may have automatically gotten it at the time) but you will need to prove when he became a US citizen, how old you were at the time and proof that you are, in fact, his child.

Yes he can still apply for you. He just needs to explain why he did not list you as his son. Worse come to worst he may be required to do DNA test to prove the relationship.