My boy friend is 20 and I'm 16 I'm carring his baby we live in Pennsylvania he has ask my dad and my dad said no we wanted to know if we can get married with out and one singing for me to get married can I get a letter from my dr saying I am with child we just want to get married

A letter from a doctor won't help. In PA, 16 and 17 year olds need consent from a parent or guardian to be married.
His age could get him in trouble, if an adult wanted to charge him with corruption of a minor or unlawful contact with a minor.
I hope you will find an adult to discuss your future with, from school to financing a child for the next 18 years.

Your boyfriend needs to worry about being charged with rape. If your sexual activity with him began before you turned 16, then he could be charged with statutory sexual assault under PA 3122.1.

If your parents tell you to stop seeing him or him to stay away, he could be charged with parental interference.

So don't even think about trying to get married without consent or running away to get married.

My advice is to tell your boyfriend to back off and try to convince your parents that you really are in love and that he will stand by you. Its good that HE asked for permission to marry. How long have your parents known you are pregnant? If you two, sprung both the pregnancy and the marriage at the same time, you may need to give your parents time to absorb this before they will consent.

But I will suggest that you wait until you are 18. Most teen marriages will fail. If he truly loves you he will wait.

I agree that you should both wait to get married. You wouldn't want to push your parents into reporting him for sexual assault of a minor. Give them time to absorb it all. Most parents only want what is best for their children. Remember that even though they are 'old' compared to you, they do know about life experience.
If this young man truly loves you: and respects your parents enough to ask, then he will wait until your old enough. 16 is to young to get married: even with expecting a child.
Good luck to you and your baby.