In the general population, there is one chance in a thousand although some estimates are as high as one in five hundred. The state of the science about autistic spectrum disorders is developing rapidly. As your daughters mature, there will be more and more information available to help them make decisions. By the time they are ready to consider parenthood, I would suggest that each go with their partner to a geneticist for counseling. As far as you and your husband go, it sounds like he has considerable fear of having another child with an autistic spectrum disorder.
Some couples decide to have another child.
Some do not. What is most important for your marriage is to understand each other and try to arrive at a decision in a way that is respectful to each other. Certainly having a child with special needs teaches us how little we have power over in life. In that sense our children teach us a power spiritual lesson. Some people go on, roll the dice, and have other healthy, typical children and feel healed by the experience.