I really, really feel for you. It is so difficult when you find that your partner has a different view of what you should do. My partner and I were agreed when we were in your situation, luckily, but the doctors told us that that was one of the biggest hurdles they had to face: when a couple couldn't agree.
I don't know what to advise you. I am someone who likes to have knowledge so I had amnio... but with that knowledge comes a terrible responsibility. At the moment your head is racing through all the possible lives that your child, DS or not, could have and how that will impact upon everyone else.
One thing that helped us was meeting a neuro paedo who explained very carefully the likely issues our baby would have if it survived and another very kind doctor who rang us at home and talked about our relationship, our lives, our health and how we thought we would cope with these issues in reality/practically.
Perhaps you can get some help together discussing these and then maybe it will help strengthen your husbands support to go ahead (if he sees he will be able to cope) without tests if you still feel that that is what you want to do or if you do go ahead and if DS is diagnosed.
That said, there is no telling how healthy any baby will be when they are born (all the things they can't discover right now, birth injuries etc.) and we all have to live with that to some extent. Once one has had a scare like yours though there is a kind of grief for the innocence of the dream of the 'perfect baby'.
The most senior consultant we met said that whatever decision one makes in these circumstances at times, and sometimes for long periods, that decision will be the wrong one. I think that was very wise.