I am so, so sorry this is happening to you. We are behind you no matter what decision you make. If you choose to let your little angel grow his wings now, know that he will be waiting for you in heaven....if you choose to carry him to term, know that his life will serve such a purpose in this world, regardless of whether or not he is viewed as "perfect", he will be perfect to you either way.
My brother was born with the cord wrapped around his neck so tightly that it cut off oxygen, he was born 4 pounds at full term, he was blind, deaf, mentally ********, and when he was 9 years old he was the size of a 9 month old baby. My mom sometimes wished that he would have just died at birth to save HIM from 9 years of a life with no sight and sound, or that there were ultrasounds back then and she could have made the choice before he was even born, but his life, however cruel it seemed to be, was a gift in it's own way. I was 2 years old when he was born, and his life softened my heart, and the hearts of other people in our family. I went to college and I'm now a nurse for adults with developmental disabilities, and five of my cousins have gone on to adopt children with special needs who would have otherwise never had a home because the were touched by my brothers life.
I'm in absolutely NO WAY trying to tell you what to do, I just don't want you to feel guilty either way. Sometimes when someone DOES choose to carry a less than "perfect" child to term, they feel a twinge of guilt, or selfishness...if that is the choice you make, do not feel that way at all because your baby will have a purpose and his life will mean something no matter what path you choose.
*hugs*