First of all, at this moment, the father has no legal rights at all (I am assuming that you are not married). Before he has any legal rights he must be adjudicated the father by a court. This requires paternity proceedings. Once he has been adjudicated the father, the court will address the "terms" of the judgment. This refers to custody, placement, child support, and other related financial issues. The father has no greater rights because he is eighteen, and you are seventeen. Incidentally, having sexual intercourse with a seventeen year-old girl is a misdemeanor. So, technically, he could be charged with a crime.
This answer is for informational purposes only. By answering this question, no attorney/client relationship is created. Although the legal information is accurate, it may not be appropriate for your situation. The best way to handle any legal problem is to seek the advice of an attorney.
Sign up to receive a 5-part series of useful information and advice about child custody law.