Well, your husband doesn't get a vote in this. He cannot make his daughter have an abortion. You need to tell him shut the hell up and get over himself.
However, you need to stop being "happy" about another "bundle of joy" in the household. This is NOT your baby. Your husband has already raised a family and he is NOT interested in going through this again in his home. Your unbridled joy is inappropriate.
Your daughter has some planning to do. She needs to start understanding point blank that life is going to be different now. She will need to actually take care of her child. NOT expect you to be the mother and her to be the sister. Not expect you to pay for everything. No parties. No dances. No going out with friends. The boy needs to go get a job and start buying diapers for the baby. She also needs to realize that the boyfriend will probably walk away. What does she think? That they'll end up married and live happily ever after? The odds are astronomical that it WON'T happen. She needs to finish school, too. There is usually an alternative school for pregnant teens - the school district will tell you what she needs to do there.
Your daughter has NO idea just how she has hurt herself here. She is still a CHILD. She had no business getting pregnant. She needs to be considering adopting this baby out. NO DOUBT part of the reason she isn't considering it is because of your excitement. There are couples out there who are trying desperately to get pregnant. They are able to provide a wonderful home with financial support and they are ADULTS, not children mooching off their parents, having babies.
Stop making this easy on your daughter. You also need to understand that she could just walk away from the responsibility of this child and leave him/her with YOU. Your husband vehemently wants a life without a baby. You may end up ALONE with your daughter and grandchild without him. Sit down and discuss the possibility of adoption with her and take yourself wanting a grandchild OUT of the picture.
I have a dear friend who was unable to have children. After years of trying and unsuccessful IVF, they decided to try to adopt. It was a 16 year old girl and her 16 year old boyfriend. My friend and her husband met with both of them. Thankfully there were no drugs involved. They adopted the baby right after she was born, and sent pictures for many years of this child to the birth mother. This baby is now a freshman in college and a wonderful young woman.
Your daughter needs to stop just thinking about the "coolness" of a baby. You need to stop thinking about how wonderful it will be to be a grandmother. Your daughter needs to grow up and have a normal life FOR NOW that includes high school graduation, college, marriage and THEN babies.
Get her to a counselor. There is still a chance that these kids will figure out that they are NOT cut out to be parents if you aren't promising them the moon to keep the child.