I think they say the risk of m/carriage with amnio is between 0.5% and 1%.
My nuchal scan came back v high risk of downs and I went with an amnio. My DH was adamant that we'd abort if it was DS, I was less certain (not sure about putting our other son first and him losing out time-wise, health problems with ds, but I wanted to stay pregnant, was confused etc...).
Amnio result was a definite "no" for ds. I still don't know what our final decision would have been if it had needed making...
However, I now have a DS2 who has autism! (and DH is as proud and in love with him as I am!). But I think he found it harder to get used to the idea, and speaking to other couples in the same position, it seems to be the husband who finds it harder and takes longer to come to terms with.
I think I'm trying to say that there's all sorts of special needs that can't be anticipated, but having an SN child does become "normal" because it's "normal" for that child (does that make sense?). My DH seemed very definite in what he said at the time, but the reality has changed his opinion - over time! Do you think this might apply to your DH? (obviously reality could hit a bit earlier in your situation).
Would it be worth "engineering" a meeting with parents of the children with downs that you know? Would DH do it in the guise of supporting you as you try to find out more?
Sorry you're in this tricky situation at the moment, hope it all works out ok, for all of you!!