First off, I'm assuming that you have had pre-natal checkups and ultrasounds? If so, did your boyfriend come along with you? If your nurse knew that the father of your child was over 18 he or she is legally suppose to turn you in to the judicial system. This actually just happened to a friend of mine. Now, I know that sounds hasty and scary but don't worry. They will do the initial court procedures but if your mother and father aren't willing to press charges against him, ultimately the court case will be dismissed. Let's face it, the judge isn't stupid. He or she knows that they can't take the father of your child to jail or else they will have to fork out money for you child support.
You mentioned wanting to change the date on your child's birth certificate. Do you knwo how much that would effect your child? They would be held back so much and it would be noticable that they would mature at a faster rate then the other children. It's not logical. Also, I doubt that it's possible or legal. Unless you want your child to be considered a bastard, the father has to sign the birth certificate. This also entitles you to child support. You're going to need that additional income with a child.
Are your parents involved with this pregnancy? Do they know that the father is over-age? Do they seem angry with it? Ultimately it would be up to them whether or not to press charges on him so those would be the questions to ask yourself. If you really think that they would, maybe it would be better to just tell the doctors that you are unsure of a child. This would keep him out of trouble but could cause trouble if in the long run he decides to skip out on you and the baby. He would have no legal claim on the child, therefor not have to pay any money too you.
It sounds to me as if you have a lot of thinking to do.
It was a little different but I was 18 when I got pregnant and my son's father was 34, no one cared. It's up to the parents to press charges.
If you try and falsify a birth certificate, I'm pretty sure that's fraud and you could go to jail. That would be stupid, don't do it.
Just put him down on the birth certificate properly and leave it at that.